A Turkish homeowner chasing his chickens through a hole in his basement during renovations came across an abandoned underground Turkish city that once housed 20,000 people.

To recapture his escaping poultry, the unidentified man knocked down the wall in the 1960s to reveal a dark tunnel leading to the ancient city of Elengubu, known today as Derinkuyu.

Derinkuyu burrowed more than 280 feet beneath the Central Anatolian region of Cappadocia, is the largest excavated underground city in the world and is believed to connect to more than 200 smaller, separate underground cities that were discovered in recent decades, Tuguides told the BBC.

Inside the subterranean city — whose entrances connect to more than 600 private homes in the modern, surface-level region of Cappadocia — researchers found 18 levels of tunnels containing dwellings, dry food storage, cattle stables, schools, wineries, and even a chapel.

“Life underground was probably very difficult,” the guide, identified as Suleman, told the outlet.

“The residents relieved themselves in sealed clay jars, lived by torchlight, and disposed of dead bodies in [designated] areas.”

The exact date the impressive city was built remains contested, but ancient writings dating back to 370 BC indicate Derinkuyu was in existence.

The city was likely originally used to store goods but was then used as a bunker to escape from foreign invaders — the dimly lit hallways were intentionally built narrow and low so intruders would be forced to stoop and enter in single file.

The doors connecting each level were blocked by half-ton boulders only moveable from the inside that contained a small hole that allowed residents to spear the confined trespassers.