-The Deepest Place On Earth- Amazing Full Documentary 2016
Category
😹
FunTranscript
00:00 ♪ ♪
00:02 Earth, a 4.5-billion-year-old planet still evolving.
00:09 As continents shift and clash,
00:12 volcanoes erupt, and glaciers grow and recede,
00:17 the Earth's crust is carved in numerous and fascinating ways,
00:22 leaving a trail of geological mysteries behind.
00:27 In this episode, the Marianas Trench,
00:31 the deepest point on Earth, is explored.
00:35 Its sheer walls cut seven miles into the Pacific Ocean.
00:40 The mystery of what created this deep, dark chasm
00:43 takes science detectives on some of the most dangerous dives
00:46 ever attempted, deep into the abyss.
00:52 Scouring the ocean floor,
00:53 scientists uncover a strange undersea world
00:57 of fiery mountains, bizarre mud volcanoes,
01:03 and the largest geological structure on Earth.
01:07 Discoveries from this unique underwater world
01:10 will revolutionize our understanding
01:12 of the powerful forces that shape not just the trench,
01:16 but the Earth itself.
01:18 ♪ ♪
01:26 Hidden deep beneath the waves of the Western Pacific
01:29 lies the Marianas Trench,
01:33 the deepest point of all the oceans.
01:39 The first step on the journey of what created
01:42 this mysterious scar in the Earth's crust
01:45 and how it continues to mold the planet
01:47 takes us back to 1872,
01:50 when a British research vessel, HMS Challenger,
01:53 set out on the first-ever mission
01:55 to map the ocean floor.
01:57 Throughout most of recorded history,
01:59 men had just assumed that beyond a certain level,
02:02 the sea was pretty flat, pretty dead, fairly lifeless.
02:07 They weren't expecting to find anything very interesting.
02:13 For four years, the Challenger crisscrossed the oceans,
02:16 covering 70,000 miles,
02:19 a third of the distance to the moon.
02:24 The crew plumbed the depths every 140 miles,
02:27 using a total of 249 miles of rope
02:31 and hundreds of pounds of lead weight.
02:35 It was tedious, backbreaking work,
02:37 but at the time, it was the only way
02:39 to measure the depth of the ocean floor.
02:42 ♪ ♪
02:46 When they got to the Western Pacific,
02:48 200 miles off the island of Guam,
02:51 the crew routinely lowered the rope for a measurement.
02:54 ♪ ♪
02:59 But the weight kept on dropping and dropping.
03:04 - It's a big surprise.
03:06 Nobody thought the ocean was this deep.
03:08 So all of a sudden, we've got scientists saying,
03:11 "Why is that?"
03:13 ♪ ♪
03:20 - Eventually, the weight struck the bottom
03:22 at 4,475 fathoms,
03:26 nearly five miles beneath the ocean's surface.
03:31 - The scientists will be going, "Wow, we found something.
03:34 And what does it mean?
03:36 Is it a little hole? Is it a big hole?
03:38 What kind of feature is it down there?"
03:40 There's a whole lot of questions you get
03:42 when you find this one spectacular reading.
03:45 ♪ ♪
03:47 - The Challenger expedition marked the birth
03:49 of modern oceanography
03:51 and provided the first crude map of the ocean floor.
03:54 ♪ ♪
03:56 It showed how the ocean floor
03:58 gently slopes away from the land
04:00 and then plummets thousands of feet
04:02 into vast, flat plains.
04:05 But the Western Pacific is different.
04:07 It drops off again into the five-mile deep hole,
04:12 a hole that blew right out of the water
04:14 the long-held belief that the sea floor
04:16 was flat and featureless.
04:18 ♪ ♪
04:21 And it spawned a mystery,
04:24 because nobody could understand
04:26 how this strange underwater feature came about.
04:30 It would be 75 years before any answers emerged.
04:34 So they took a revolutionary new technology, sonar,
04:37 to push the investigation forward
04:39 to the next crucial stage.
04:41 ♪ ♪
04:48 Sonar was first developed in the early 1900s
04:51 and then perfected during the 1940s
04:54 to detect submarines lurking in the deep.
04:57 ♪ ♪
05:02 The system works by pumping sound waves through the water.
05:06 The waves bounce off solid objects
05:09 and are reflected back to a detector.
05:12 By measuring the time it takes for the sound waves
05:14 to bounce back, scientists realized
05:17 they could build a remarkably accurate picture
05:19 of the world beneath the waves.
05:22 The world's major navies spend a lot of time and effort
05:24 developing submarine hunting technology.
05:28 Then the hydrographers discover that you can use this
05:31 to chart the bottom of the sea,
05:33 and it's an awful lot cheaper and easier
05:35 than using large numbers of sailors pulling on ropes.
05:38 ♪ ♪
05:42 In 1951, a British Navy research ship
05:45 returned to the deep hole found by the Challenger expedition.
05:49 ♪ ♪
05:51 But this time, they were armed
05:53 with sophisticated new sonar equipment.
05:56 ♪ ♪
05:59 And the results were amazing.
06:03 Detailed sonar maps revealed that the deep hole
06:06 in the Pacific Ocean floor isn't a hole at all,
06:09 but part of a massive trench 30 times deeper
06:13 than the Empire State Building is high.
06:15 ♪ ♪
06:18 It runs twice the length of California,
06:21 1,500 miles from the southeast of Guam
06:24 to the northwest of the Mariana Islands.
06:27 ♪ ♪
06:30 People were probably astounded by what they were seeing
06:33 because clearly the ocean floor
06:35 had enormous changes in relief,
06:37 was very mountainous in some places,
06:39 had great deeps in other places.
06:42 To a geologist, this would be extremely exciting.
06:46 Even within the trench itself, there are remarkable variations.
06:51 At its southern end lies the greatest surprise of all.
06:55 ♪ ♪
06:58 The sea floor drops down another two miles
07:01 to its lowest point,
07:03 a staggering seven miles beneath the waves.
07:06 ♪ ♪
07:09 Scientists had discovered the deepest part of the oceans.
07:13 Even today, it is the lowest known point on the planet.
07:18 ♪ ♪
07:20 They named this part of the trench the Challenger Deep
07:23 after the ship that discovered it.
07:25 To get a sense of just how deep trenches are,
07:28 if we take the height of Mount Everest,
07:31 we would still have about a mile of water above us
07:34 before we get to the ocean's surface.
07:38 But how the Marianas Trench was formed remained a mystery.
07:44 Investigators decided the best way to find the answer
07:47 was to dive to the bottom of the trench...
07:50 [water splashing]
07:52 ...to see for themselves the lowest point on the planet,
07:55 the Challenger Deep.
07:58 But they faced a major problem.
08:02 At the bottom of the trench, they would have to contend
08:04 with pressure 1,000 times stronger than at the surface.
08:08 That's the equivalent of being squeezed on all sides
08:11 by the weight of 50 jumbo jets.
08:14 ♪ ♪
08:17 To demonstrate the effects of such pressure,
08:20 scientists use a dummy head.
08:23 Today what we're gonna do is actually put
08:26 one of these styrofoam wig heads in the pressure chamber
08:31 and expose it to the pressure we would see
08:34 in the Marianas Trench.
08:36 That's about 16,000 psi.
08:40 A human skull would be crushed to a pulp,
08:43 but the rubbery head will only have all the air squeezed out.
08:47 ♪ ♪
08:50 Wow, that's smaller.
08:53 And here's what the original size was,
08:56 just for comparison.
08:58 [chuckles]
08:59 Quite dramatic.
09:01 Pretty stark difference between something
09:04 that hasn't been seven miles deep in the ocean
09:06 and something that has.
09:08 Glad I'm not going there. [chuckles]
09:10 At the Mariana Trench, human life is impossible.
09:14 We're not equipped to resist those kinds of pressures,
09:19 and so it's necessary to protect humans
09:22 from that type of an environment.
09:25 The challenge to engineers was how to accomplish this.
09:30 In 1953, Swiss scientist Auguste Piccard
09:34 designed the Trieste,
09:36 a pioneering vehicle that could withstand
09:39 the crushing pressures.
09:41 ♪ ♪
09:46 The submersible was dominated by a 50-foot-long hull,
09:50 filled with light aviation gasoline
09:52 and lead weights to control buoyancy.
09:55 Slung underneath it was a tiny 6-foot spherical cabin
09:59 with 5-inch-thick steel walls.
10:02 ♪ ♪
10:07 Finally, after seven years of modifications
10:10 and manned test dives no deeper than 3 1/2 miles,
10:14 the Trieste was ready to attempt the seven miles
10:17 to the bottom of the trench.
10:20 The commander of this perilous undertaking
10:22 was U.S. Navy Lieutenant and deep-sea explorer Don Walsh.
10:27 I know the astronauts that go through this all the time.
10:30 "Why do you have to be there?
10:31 "Why can't we just put up a robot to do things?"
10:34 We've got to be there because that's what we do.
10:38 Only a few officers and scientists knew
10:40 about the risky mission, which was launched in January 1960
10:44 from the western Pacific island of Guam.
10:48 Guam in those days was kind of a backwater.
10:51 It was just right for us because we were trying to do
10:54 this project sort of out of sight
10:57 because we weren't too sure it was going to work.
11:00 The Navy just didn't want to be embarrassed
11:02 by a failed science spectacular.
11:05 ♪ ♪
11:08 Accompanying Walsh was the son of the Trieste designer,
11:11 engineer and oceanographer Jacques Piccard.
11:15 The two men would spend the next nine hours
11:17 squeezed inside the cramped sphere.
11:21 And we had 20 cubic feet of space inside.
11:25 That's about the same as a household refrigerator.
11:28 And the temperature was almost that cold inside.
11:33 It was a drama.
11:35 The story of how the Marianas Trench came to be
11:40 is beginning to take shape.
11:44 In 1874, British surveyors were the first to discover
11:48 a five-mile deep hole in the ocean.
11:51 75 years later, sonar mapping revealed the hole
11:54 to be a vast 1,500-mile-long trench
11:58 with the deepest part seven miles
12:00 beneath the surface waves of the Pacific.
12:04 To gather further evidence, two courageous men
12:07 were about to undertake the most dangerous dive in history.
12:11 They would venture into the abyss
12:14 and go to the bottom of the Marianas Trench.
12:18 The Marianas Trench is one of the most remote,
12:23 inhospitable places on Earth.
12:29 In January 1960, two deep-sea explorers,
12:33 Don Walsh and Jacques Piccard,
12:35 plunged into its depths on board the submersible, the Trieste.
12:40 At a speed of just three miles per hour,
12:49 they began their slow descent into the twilight zone.
12:56 By 3,000 feet, the darkness was total.
13:01 The only illumination was from the Trieste's powerful lights.
13:06 The depths we were operating at, it was always black.
13:11 The only thing that lit up the abyss
13:13 was the bioluminescence from animals and plankton.
13:18 Like fireflies, they carry their own light sources with them.
13:24 Encased in their five-inch-thick steel sphere,
13:27 Walsh and Piccard quickly passed
13:29 their test dive record of 18,000 feet.
13:33 Everything appeared to be going to plan.
13:37 At the rear of the cabin, the crew were protected
13:40 by a double layer of glass.
13:42 But two hours into the dive, the outer pane cracked.
13:47 We had a very big bang.
13:52 We didn't know what it was. We were about 20,000 feet.
13:55 And we looked around and checked everything.
13:58 Every square inch of their tiny life-supporting capsule
14:02 was fighting back eight tons of pressure.
14:06 With the outer pane broken, the only thing between the men
14:10 and instant death was a single pane of glass.
14:13 If the inner window had cracked,
14:15 we would have been instantly dead,
14:18 maybe even before we knew it.
14:21 But incredibly, the inner pane remained watertight.
14:26 Walsh and Piccard decided to continue the descent.
14:30 After a tense, claustrophobic four hours and 48 minutes,
14:36 they approached the bottom of the trench,
14:38 only to be startled by movement on the sea floor.
14:42 Just before we landed, we saw a flatfish about a foot long.
14:49 And that's a bottom-dwelling fish,
14:51 so if you see one, there are others.
14:55 Nobody expected to see life at these crushing depths,
14:59 but it meant the explorers had reached their goal--
15:02 the very bottom of the Marianas Trench.
15:10 The depth gauge, with a reading of 35,800 feet,
15:13 nearly seven miles below the surface,
15:16 confirmed the sonar findings.
15:19 Squeezed inside their bubble of breathable air,
15:27 the two explorers were closer to the Earth's center
15:30 than man had ever been.
15:32 We took a self-portrait.
15:35 That's the picture that you see.
15:38 Said we're going to do it, and we did it.
15:41 But there was work to be done.
15:45 Walsh and Picard wanted to make detailed observations
15:48 of the enormous trench.
15:52 Unfortunately, the Trieste stirred up a cloud
15:54 of fine, powdery sediment from the sea floor
15:57 that obscured their view.
15:59 It's like being in a bowl of milk at that point.
16:04 So realizing we weren't going to see anything,
16:06 we decided to go on back up to the surface.
16:09 Off the island of Guam, the Trieste surfaces
16:12 after a descent into the Marianas Trench.
16:15 After nine grueling hours underwater,
16:18 Walsh and Picard returned to the surface on January 23, 1960
16:23 and officially entered the record books
16:25 for the deepest dive of all time.
16:28 To this day, their extraordinary feat
16:31 has never been repeated.
16:33 The mission was a success,
16:39 but the mystery remained.
16:43 Geologists still didn't understand
16:45 what could have formed the immense trench.
16:49 And if they couldn't find the answer inside the trench,
16:53 they would have to look elsewhere.
16:56 Perhaps there was something somewhere on the ocean floor
17:01 that might explain the trench's origins.
17:08 Throughout the '50s and '60s, a team of geologists
17:11 led by Princeton's Harry Hess compiled sonar data
17:14 from all of the world's oceans.
17:17 It was as though they had pulled out a giant plug
17:23 to drain away all the water and expose the ocean floor.
17:28 Their maps revealed that the Marianas Trench
17:31 is just a tiny fraction of a network
17:34 of enormous underwater canyons
17:36 stretching right around the planet.
17:39 But that wasn't all.
17:41 Running parallel to the trench,
17:43 on the other side of the Pacific,
17:44 the map showed a giant underwater mountain range,
17:47 the East Pacific Ridge.
17:50 And this, too, is part of a global network,
17:53 a 40,000-mile-long chain of mountain ranges
17:56 that ring the globe like the seams of a baseball
18:00 to make the largest geological feature on Earth.
18:05 It was a major development in the investigation,
18:08 one that scientists hoped might explain the trench's formation.
18:13 The next step was clear.
18:17 Investigators needed to understand
18:19 whether there was a connection
18:20 between the trench and the East Pacific Ridge.
18:32 The breakthrough came from the unlikeliest of sources.
18:37 During the Cold War,
18:39 the U.S. built a vast network of underground seismometers
18:43 to pick up atomic bomb testing around the world.
18:47 Inadvertently, the seismometers
18:50 also detected naturally occurring earthquakes.
18:55 When geologists plotted these on a map,
18:58 a pattern emerged.
19:02 The earthquakes were clustered
19:04 along the ocean's ridges and trenches.
19:08 It was a discovery that transformed
19:10 our understanding of the Earth.
19:13 Geologists realized the friction that causes earthquakes
19:16 comes from movements that must be occurring
19:19 deep beneath the ridges and trenches.
19:22 With this great investment in seismology,
19:24 it became possible to locate very precisely
19:28 where earthquakes had occurred.
19:30 And it was these things, the precise location,
19:33 the depth, and the motion,
19:35 that really gave the outlines of plate tectonics.
19:39 It was the birth of an extraordinary new theory.
19:44 The solid layer of rock, the crust,
19:47 on which the land and ocean sits,
19:49 is broken up into a series of vast slabs
19:53 that geologists call tectonic plates.
19:56 It's these plates that are moving,
19:58 grinding past each other, and triggering earthquakes.
20:04 The underwater ridges and trenches
20:08 sit on the boundaries between tectonic plates.
20:12 The East Pacific Ridge and the Marianas Trench
20:15 lie on opposite edges of the Pacific Plate.
20:20 The journey to discover what formed the Marianas Trench
20:23 is accumulating additional evidence.
20:26 The Trieste dived to the bottom of the trench
20:30 and confirmed that it is the deepest point on the planet.
20:35 Sonar maps then revealed the East Pacific Ocean Ridge
20:39 running parallel to the trench.
20:42 To solve the mystery of the Marianas Trench,
20:46 investigators needed to find out
20:48 exactly what was happening at the East Pacific Ridge,
20:52 and that meant exploring these vast mountains.
20:55 8,000 feet underwater.
20:59 The pieces of the Marianas Trench puzzle
21:06 are falling into place with the knowledge
21:08 that it lies on the western edge
21:10 of the Pacific tectonic plate.
21:14 On the opposite side of the plate
21:16 lies the East Pacific Ocean Ridge,
21:19 part of an enormous chain of underwater mountain ranges
21:23 that ring the globe to create
21:25 the largest geological feature on Earth.
21:29 Scientists had a hunch that this colossal ridge
21:32 might help explain how the trench was formed.
21:37 And they found a major clue halfway around the globe,
21:41 where the ridge passed beneath the middle
21:43 of the Atlantic Ocean.
21:47 During the Cold War, the U.S. Navy developed a new technique
21:52 to combat Soviet submarines.
21:55 They scanned the seas with a tool called MAD,
21:59 a magnetic anomaly detector,
22:01 which could pinpoint steel hulls lurking in the deep.
22:06 But they stumbled across something else.
22:11 Running parallel on either side of the ridge,
22:13 they found strange stripes of magnetic rocks
22:16 alternating positive and negative
22:18 away from the ridge's peak.
22:21 Here's the mid-Atlantic ridge coming down through here.
22:25 Almost perfectly symmetric on either side of that
22:28 are these white and black stripes.
22:31 These have often been called zebra stripes.
22:36 Geologists know that the Earth is like a giant magnet
22:39 with a north and a south pole.
22:42 But the magnetic poles aren't fixed.
22:45 Every 300,000 years or so,
22:47 the magnetic field suddenly flips 180 degrees.
22:54 When the field flips,
22:55 a compass that was previously pointing north
22:58 will swing to the south.
23:00 This reversing of the Earth's magnetic field
23:03 is a very interesting and exciting
23:06 but very puzzling phenomenon for geophysicists to explain.
23:11 Scientists think this reversal explains the stripes
23:15 on either side of the ocean ridge.
23:20 In the 1960s, geologists discovered
23:22 that molten volcanic rock known as magma
23:25 swelled up from deep underground
23:27 to create the ridges in the Atlantic and Pacific.
23:32 As magma wells up between the tectonic plates,
23:36 it pushes the sea floor up
23:38 and forms the colossal mid-ocean ridge
23:40 thousands of feet high.
23:46 When the rock is hot and molten,
23:48 its magnetic minerals line up with the north-south direction
23:52 of the Earth's magnetic field.
23:54 As the magma cools, the minerals are locked in position.
24:02 These rocks act as a permanent record
24:04 of the magnetic pole's location when the rocks were formed.
24:11 As more and more magma is forced up,
24:13 the old crust is pushed away from the ridge
24:16 and records the reversals in the Earth's magnetic polarity.
24:20 If you have reversals of magnetic polarity,
24:22 then the sea floor acts sort of like a tape recorder
24:25 and records these changes in magnetization.
24:28 Then the pattern of magnetic stripes
24:30 allows people to calculate the speed
24:33 at which the plates are moving apart.
24:39 The zebra stripes are proof that over time,
24:42 the sea floor in both the Atlantic and the Pacific
24:45 is spreading away from the ridges
24:47 at a rate of more than two inches a year.
24:51 But geologists needed proof that magma created the ridge.
24:57 If red-hot molten rock is forming the enormous mountain
25:00 range in the Pacific, the surrounding water
25:02 should be warm.
25:03 In 1977, a team of scientists set out
25:11 to discover whether this warm water really existed.
25:14 Dudley Foster was the pilot for these historic dives.
25:22 It's been an exciting occupation because you're
25:24 on the cutting edge of science, new discoveries all the time.
25:30 Every cruise, there's a new group of scientists
25:32 with new scientific objectives, and there's
25:34 the exploration and the discovery.
25:37 And that's really what puts the thrill in the job.
25:40 For weeks, the crew scanned the undersea mountains
25:49 without success.
25:52 And then they hit the jackpot--
25:54 a bizarre pillar of rock spewing hot, toxic gas.
26:03 And we saw the water was sort of shimmering,
26:09 sort of like a bubbling in a glass teapot or something.
26:15 We stuck the temperature probe in there
26:17 and measured 38, 39 degrees Fahrenheit, which
26:21 was really amazing because the ocean's a huge heat sink.
26:25 So to see something warm like that was kind of startling.
26:30 In these pillars of rock, the expedition
26:32 had found the heat from the magma surging up
26:35 from deep inside the Earth.
26:39 It wasn't warming the water evenly along the ridge.
26:43 It was channeled up through strange hydrothermal vents.
26:48 When you make these discoveries, you
26:50 don't know how significant they are.
26:53 The true significance of them maybe takes several years
26:57 to appreciate.
26:59 And this was one of those times.
27:00 For the investigation into the Marianas Trench,
27:06 these vents are a decisive piece of evidence.
27:08 They confirm that magma is continually
27:14 creating new crust at the Pacific Ocean Ridge.
27:18 And magnetic zebra stripes prove that old crust
27:21 is pushed away from the ridge towards the other side
27:24 of the Pacific Plate, towards the Marianas Trench.
27:27 But this presents scientists with a puzzle.
27:33 If new crust is being created at the ocean ridge
27:36 and the Earth isn't expanding, then the old crust
27:39 must be disappearing somewhere else.
27:42 The reason that there is not getting bigger
27:45 with seafloor spreading is because the same amount
27:47 of seafloor is being destroyed in the Pacific.
27:51 Something in the Pacific Ocean is devouring the seafloor.
27:56 And all the evidence points to the Marianas Trench.
28:00 In the hunt to discover what formed the Marianas Trench,
28:08 scientists now know crust created at the ocean ridge
28:12 is being devoured somewhere and by something
28:15 in the Pacific Ocean.
28:17 They suspect the Marianas Trench is involved.
28:24 But the proof would come not from the trench,
28:30 but from these, the Mariana Islands,
28:35 a chain of volcanoes that break through the ocean's surface
28:39 200 miles west of the trench.
28:43 Scientists noticed the island chain mirrors
28:46 the trench's exact shape.
28:48 This led them to think the trench was responsible
28:51 for the island's creation.
28:53 If you see pictures of the Marianas Trench,
28:58 it's curved and the line of volcanoes that it generates
29:01 is curved exactly parallel to it.
29:03 Geologists believe that the trench formed the volcanoes
29:08 via a process called subduction.
29:12 Subduction occurs where two tectonic plates collide.
29:16 As they grind past each other, the heavier plate
29:19 is pushed beneath the lighter plate.
29:22 The descending plate is forced down
29:24 into the Earth's intensely hot interior called the mantle.
29:28 It takes with it water and sediment built up
29:30 over millions of years.
29:32 Volcanoes form above subduction zones
29:35 not because the Earth is hotter there,
29:37 but because this is where we're taking the water that
29:40 once was in the ocean.
29:42 It's taken into the mantle and gets sweated out,
29:45 causes the mantle to melt, and this magma is what then rises
29:48 and erupts explosively out these volcanoes.
29:51 The water in the sediment forces magma to swirl up
30:00 and push through the plate above.
30:03 And when it breaks the surface, it creates volcanoes,
30:07 like the volcanoes that formed the Mariana Islands.
30:10 It was subduction that formed the islands west of the trench
30:18 and gave investigators the breakthrough
30:20 they'd been looking for, because here at last was
30:25 a process powerful enough to create the Marianas Trench.
30:28 As the descending plate dives down, it digs into the mantle.
30:36 Here, the colliding plates form a trench, a giant crease
30:40 in the ocean floor.
30:41 It seemed that scientists had finally explained
30:46 how the trench was formed.
30:48 There was just one problem, a very large problem.
30:56 Around the world, subduction zones
30:58 cause violent earthquakes and catastrophic tsunamis.
31:05 We know subduction is happening because
31:07 of the active earthquakes, and these are
31:09 the most devastating earthquakes.
31:11 This is the earthquake that generated the tsunami in Sumatra.
31:17 Also, the other very large earthquakes in Alaska and Chile.
31:22 But the Marianas Trench, the deepest subduction zone
31:29 in the world, hasn't caused the devastating earthquakes
31:32 that records began in the 17th century.
31:35 Investigators needed to know why.
31:39 Ah, that's a $60,000 question.
31:42 They hoped the trench's shallower western edge
31:48 might provide the answer.
31:50 Here, they found an intriguing chain of underwater hills
32:00 two miles below the surface of the sea.
32:02 Engineers drilled down into the hills
32:10 and collected core samples.
32:11 And when the scientists analyzed the samples,
32:20 they discovered the hills were actually volcanoes,
32:24 and they spewed out not lava, but mud.
32:29 The fine, powdery mud is made up of a soft type of rock
32:33 that has been ground up in the subduction zone.
32:36 It seemed this soft rock might explain
32:40 why there have been no major earthquakes
32:42 at the Marianas Trench.
32:44 Everybody has a sense of what a volcano is,
32:48 but not all volcanoes erupt igneous rocks.
32:51 There's some volcanoes that erupt mud,
32:54 and a certain kind of, an unusual kind of mud
32:57 in the Marianas is made out of serpentine.
33:00 And serpentine is a very weak rock,
33:03 and it can be scratched with a knife or something like that.
33:06 Investigators realized the grinding plates
33:11 crushed the soft rock to form a lubricating mud
33:14 that prevents large earthquakes.
33:17 Then the mud bubbles up to the ocean floor,
33:21 where it forms the strange mud volcanoes
33:24 found along the trench's western edge.
33:27 Other parts of the world, like the Andes or maybe Indonesia,
33:30 you've got two plates that are grinding together,
33:33 and one of the plates is quite strong,
33:35 and it takes a big earthquake to rupture that plate interface.
33:39 But if these rocks are weak, like they are in the Marianas,
33:42 where you've got these serpentinites,
33:44 those are very weak, and it doesn't take much energy at all
33:47 to get the two plates to glide one past the other.
33:53 At last, geologists had discovered
33:56 what created the Marianas Trench.
33:59 50 million years ago, the Pacific Plate
34:03 slipped under the edge of the Philippine Plate.
34:06 As it bent and dived into the Earth's mantle,
34:09 it formed the colossal Marianas Trench.
34:12 And the plate is still moving.
34:16 Like a giant conveyor belt,
34:18 the Earth's crust travels slowly across the Pacific Plate
34:22 from its birthplace in the East Pacific Ridge
34:25 to its graveyard 10,000 miles away in the Marianas Trench.
34:29 Today, the Pacific Plate's movement can be tracked in real time.
34:35 Confirmation has come from GPS technology,
34:40 where we can actually put a transmitter on an island
34:44 and come back year after year and actually follow it
34:47 moving a few centimeters a year towards the trench.
34:51 It's devouring the crust at a rate of three inches a year,
34:55 about as fast as a human fingernail grows.
34:58 Every four million years,
35:03 it swallows an area the size of the United States.
35:06 By consuming the crust created at the Pacific Ocean Ridge,
35:10 the ravenous Marianas Trench is the world's largest recycling plant.
35:16 (music)
35:18 But there was one remaining and major piece of the puzzle to find.
35:26 Scientists still didn't know why it is the deepest trench on Earth.
35:31 They suspected the age of the seafloor at the bottom of the trench
35:36 may provide the answer.
35:38 It turns out there's a really strong relationship
35:41 between the age of the seafloor and its depth in the water.
35:45 In 1999, a team of deep sea drillers returned to the trench to collect core samples.
35:54 One great thing about drilling the Stochen crust is we actually got pieces of it.
36:00 So we're holding in our hands here the material that's actually getting subducted at the Marianas Trench,
36:06 and it turned out to be 170 million years old.
36:10 So we can say with confidence that's the oldest ocean floor
36:13 before it's getting swallowed up in the mantle at the trench.
36:16 But why is this piece of rock the oldest on the ocean floor?
36:22 The seafloor at the Marianas Trench is so old because it's been so long since it was born.
36:28 So it was born in the equivalent of the eastern Pacific today,
36:32 and it's just been going on longer than any other place in the oceans before it's been subducted.
36:39 The Pacific Plate is the planet's largest tectonic plate,
36:43 covering an area 11 times the size of the United States.
36:47 When crust bubbled up at the ridge 170 million years ago, it was light and buoyant.
36:58 But as it traveled 10,000 miles across the plate, it cooled and became compact and dense.
37:06 Over millions of years, the dense crust got heavier and began to sink into the mantle below.
37:12 Scientists realized that because the crust at the Marianas Trench is the oldest ocean crust,
37:23 it's also the heaviest and so has sunk deeper into the mantle than any other area of ocean crust.
37:33 Here at last was the explanation for the trench's extraordinary depth.
37:38 The picture of the Marianas Trench is almost complete.
37:43 Volcanic islands mirroring the trench's exact shape lead scientists to believe it runs along a subduction zone.
37:52 And slippery mud volcanoes explain why it doesn't create large earthquakes.
37:59 One question remains unanswered. Towards the trench's southern end,
38:04 the vast chasm drops a further two miles to its lowest point, the Challenger Deep, seven miles beneath the waves.
38:13 The question is, what makes it plunge so deep?
38:19 The investigation into the Marianas Trench has one final puzzle to solve.
38:29 At the trench's southern end, the sea floor plummets a further 10,000 feet
38:35 into a seven-mile deep chasm called the Challenger Deep.
38:40 It's the lowest point on the planet.
38:44 But so far, scientists have been unable to explain why this one section of the trench is so deep.
38:53 Now, they believe the shape of the descending tectonic plate may hold the answer.
38:59 The Challenger Deep, in addition, is a little bit deeper because of some peculiarities relating to how the slab that's going down is behaving.
39:09 A narrow slab of crust has torn away from the Pacific plate's descending edge.
39:16 Well, it's basically got to do with how the slab pushes the mantle out of the way.
39:22 Where you have a narrow slab, like you have at the Challenger Deep, it can sink almost vertically
39:28 because the mantle that it's trying to displace can move around out of the way.
39:33 Investigators have finally unraveled the mysteries of the Marianas Trench.
39:40 And in the process, they've made a discovery with implications that stretch far beyond the trench itself.
39:51 [music]
39:55 Studying the ocean ridges led geologists to believe that magma welling up at the ridges was pushing the plates apart.
40:04 But the exploration of the Marianas Trench has changed this idea forever.
40:13 People used to think that maybe the magma would kind of push the plates apart.
40:19 And that idea is largely discounted now.
40:23 As the ocean crust travels from the Pacific Ocean Ridge to the trench, it changes from a buoyant, red-hot magma into a colder, denser, and heavier crust.
40:35 The plate's leading edge becomes so heavy that it drags the rest of the plate along behind it.
40:46 The heavy, cold plates at the trenches are sinking down into the mantle and pulling the plates apart at the ridges.
40:56 And the magma just passively fills in the gaps.
41:00 The investigation into the Marianas Trench has revolutionized our understanding of how the Earth's plates move.
41:15 We now know a worldwide network of subduction zones drag tectonic plates around the globe,
41:22 powering the movement of continents over millions of years, and moving the very Earth we stand on.
41:30 The plates that are moving fastest on the Earth are the ones that have all the trenches.
41:40 The Pacific Plate is the fastest moving of the nine major plates on the planet because it is surrounded by dozens of destructive trenches like the Marianas.
41:49 They are consuming the ocean crust faster than the ocean ridge can produce it.
41:55 Over millions of years, the Pacific Plate will shrink until sometime in the distant future, the largest ocean on Earth will disappear.
42:08 Australia will crash into the United States, reshaping our planet.
42:12 Perhaps one day, downtown Seattle will compete for real estate with a suburb of Sydney, Australia.
42:20 And all because of subduction zones like the Marianas Trench.
42:31 But for all its significance, man has only ever dived to the bottom of the trench once, and there are no immediate plans to return.
42:39 Imagine asking someone, "What is the flora and fauna of California?"
42:51 And saying that someone spent ten minutes there, picked up two ants, come back, and said they've sampled California.
43:00 That's probably how well we know the Marianas Trench.
43:03 To date, less than 5% of the world's oceans have been explored.
43:12 But only by returning to the ocean's very deepest reaches will we fully comprehend the incredible forces that recycle and rebuild our world.
43:25 The way I like to think of it is that ocean exploration leads to new research questions.
43:33 And if we don't have exploration, we don't even know the right questions to ask.
43:41 It is now known what a geological wonder the Marianas Trench is.
43:51 Since this deep chasm in the Earth's crust was first discovered with a length of rope and a lump of lead more than a century ago, evidence has piled up.
44:00 A record-breaking dive to the lowest point on Earth.
44:04 Giant undersea mountain ranges with bizarre magnetic zebra stripes.
44:10 Proof that the ocean crust is spreading towards the hungry Marianas Trench, lined with slippery mud volcanoes which prevent devastating earthquakes.
44:21 And the planet's oldest ocean crust, the reason that the Marianas Trench is the deepest point in the oceans.
44:29 In the darkest and most remote place in the world, scientists have added to their knowledge about the powerful forces that contribute to the dynamic story of our planet.
44:42 [music]
44:48 [music]
44:50 [BLANK_AUDIO]