Imagine an entire city built underground and in the stone and you have the Cappadocia region of Turkey. And when I say city, I mean an area big enough for thousands of people plus livestock.
Built during times of war and persecution, these cities could house the people of the region until the invaders left.
The underground area we visited went down seven levels and had its own well. There were even churches. Another fun fact, they used round doors that rolled into place and could only be opened from the inside.