Welcome to Derinkuyu, an underground city that once housed up to 20,000 people. In the Cappadocia region, famous for its cave dwellings and underground villages, Derinkuyu stands out for sheer size and complexity. Locals began digging in the 500s BCE. The city consists of over 600 doors, each of which can be closed from the inside. Each floor could be closed off as well. And just to make attacking completely impossible, the entire city was deliberately built without any logic. Its maze-like layout makes navigating the city nightmarish for unfamiliar invaders.