Underground city of Derinkuyu

In 1963, an inhabitant of Derinkuyu (in the region of Capadocia, central Anatolia, Turkey), was demolishing a wall of his house-cave, and discovered astonished that behind the wall was a mysterious room that never had been seen; this room took to another one, and this one to another one and another one… By chance the underground city of Derinkuyu was discovered, whose first level could be excavated by hititas around year 1400 a.C.

More on Derinkuyu:
http://en.wikipedia.org/wiki/Derinkuyu_Underground_City
http://www.cappadociaturkey.net/derinkuyu_underground_city.htm