Underground City in Turkey Big Enough for 30,000 People
Derinkuyu, is an underground city, located in Turkey. Believed to have been built somewhere between the 8th-7th centuries BC - it spans eleven floors and is about 85m deep. Stone is not able to be carbon dated, so the exact date of construction is unknown.
This underground Spanning eleven floors with a depth of 85m, this ancient city can accommodate anywhere between 35,000 and 50,000 people. It contains modern day amenities such as family rooms, communal areas, stables, churches, wine and oil presses, chimneys to bring fresh air, wells to bring water, a school complete with study rooms, and makeshift tombs - all completely underground.
There is much speculation as to why this city was built, and lots of mystery shrouds the construction as the underground city features various doors on each level that close from the inside sealing it off completely from any "invaders". It is unclear who they may have been hiding from, and The History Channel even featured Derinkuyu on an epside of (my personal favorite show) Ancient Aliens.
The world is quite a fascinating place, which honestly never ceases to amaze me.