Imagine a city hidden underground, out of sight, and safe from harm. This is Derinkuyu Underground City in Cappadocia, Turkey. Built over 1,000 years ago, it was a refuge for early Christians during times of persecution. With room for up to 20,000 people, it was large enough to house an entire community. It had all the amenities of a city above ground, including workshops, kitchens, stables, and even a chapel.