Derinkuyu is an interesting place. It's located in Turkey, and in 1963, it was serendipitously discovered by a man who knocked down a wall in his house.
Today, the underground metropolis is accessible to tourists. While it's a heavily-visited attraction, traffic levels don't reach the levels they did when this city was in use. It's thought that at one point, this system was able to accommodate up to twenty-thousand people.
The map above is in Spanish, but it nonetheless illustrates how intricate these pathways are. Check out the video below for some awesome footage, and for a glimpse into what it's like all the way down there. Let us know what you thought in the comments below, and be sure to share it with your friends on Facebook!