The world’s largest underground city housed more than 20,000 people.

I've been there, it's amazing!
Building an 18-level underground city may sound like the project of an advanced future society, but the world’s largest underground metropolis was actually constructed around 1200 BCE. The ancient subterranean city of Elengubu, now called Derinkuyu, lies in the semi-arid region of Cappadocia in modern-day Turkey. Tourists can meander tunnels and walk among subterranean rooms, stables, schools, wineries, and even a chapel — all underground. At its peak, during Islamic raids on the Byzantine Empire in the seventh century, Derinkuyu housed up to 20,000 people.