The region around Cappadocia, Turkey has the remains of several large underground cities, eg Derinkuyu.

(Wikipedia quote)

Cappadocia contains several underground cities (see Kaymaklı Underground City), largely used by early Christians as hiding places before Christianity became an accepted religion. The underground cities have vast defence networks of traps throughout their many levels. These traps are very creative, including such devices as large round stones to block doors and holes in the ceiling through which the defenders may drop spears.