In the European country Georgia you can find the beautiful Cave City of Vardzia. This is actually a cave monastery from year 1185 that is dug into the side of a mountain in southern Georgia. The purpose with the monastery was to protect citizens from the Mongols, and it could fit a lot of people because it consisted of around six thousand apartments.
It was actually a woman that wanted this monastery built; Queen Tamar was on the throne of Georgia in 1185 at a young age, and she was threatened by many because of her gender, but she ended up ruling over Georgia for 29 years.
Today the monastery looks like a place that has been used in Lord of the Rings, it is mysterious but still very beautiful. Unfortunately it has been destroyed a bit by an earthquake that managed to rupture the cave system inside the monastery, and some parts even collapsed. If you thought that the place is abandoned, you are wrong; a group of monks are staying there to guard its history. One of many other caves in the world is the amazing Crystal Cave 1000ft under the Mexican desert.