The Amazing Ancient Underground City You (Probably) Never Heard of
The underground city at Derinkuyu is neither the largest nor oldest, but its 18 stories make it the deepest. The city was most likely used as a giant bunker to protect its inhabitants from either war or natural disaster.
Wiki on Derinkuyu. Supposedly 2700 years old.
How many more mysteries like this are hidden underground? There are of course, many accounts of "DUMBs"-- Deep Underground Military Bases, where extra-terrestrials are hidden or hiding, such as in Dulce, New Mexico.