Pictures have been released of a hidden, 18-storey underground city that once housed up to 20,000 people in Turkey.

The discovery was made in 1963 when a man who knocked down a wall in his basement in the region of Cappadocia stumbled across a secret room.

Now photos of the maze of kitchens, stables, churches, tombs and schools up to 18 metres beneath the earth have been revealed.

(Rex (Rex)

Believed to have been created during the Byzantine era in 780-1180 AD, the city of Derinkuyu was likely used as a bunker to protect inhabitants during wartime or from natural disasters.

The city’s security system was complex with stone doors able to close from the inside to block intruders from entering, with each storey being closed off individually.

While only roughly half of the secret city is accessible, it is connected to other underground communities by tunnels that can stretch for miles.

Derinkuyu underground city, to the south of Nevşehir, where the new settlement was discovered (Creative Commons / Nevit Dilmen)