According to Ancient Greek literature, the origins of Derinkuyu caves date back to prehistoric times when Phrygians of Helleno–Pelasgic origin built them out of the soft volcanic rock of Cappadocia, a historical region in Central Anatolia. The Phrygians, who later became Christians, expanded the underground caves to protect themselves during Arab-Byzantine Wars in 1st and 2nd centuries and the Mongolian incursions of Timur in the 14th century. When the city fell to the Ottomans, the underground caves were used as a refuge for the Cappadocian Greeks until 1923 when the Christian inhabitants were expelled during the population exchange between Greece and Turkey. The tunnels were soon abandoned and long forgotten until in 1963 when a man was renovating his subterranean home and a wall caved in revealing a hidden room. Further exploration showed that it was a deep labyrinth of tunnel networks.

The Derinkuyu underground city is located under the town and district of the same name in Nevşehir Province, Turkey, and 40 kilometers from Goreme, another historical town famous for its “fairy chimney” rock formations.

Cappadocia is home to various underground cities including Derinkuyu and Kaymakli with vast defense networks and traps such as large round stone doors throughout their many floors. 

At 60 to 85 meters (200 to 280 feet), Derinkuyu is the deepest underground city of them all in Cappadocia, while Kaymakli is the widest. The two cities are connected by an eight kilometer (five mile) tunnel.

There are over 600 doors to the city that are hidden on the surface in the courtyards of their homes. 

The doors are usually rollable, disc-shaped ones that could only be opened and closed from the inside. It also has an early example of “peephole” through which the inhabitants could see or shoot arrows. 

The city contains all types of rooms including stables, cellars, storage rooms, water tanks, wine and oil presses, refectories, and chapels. There are also niches for oil lamps. 

The city could accommodate as many as 20,000 people and still have enough space to house horses, goats, sheep, fowls and their chicks, as well as their fodder. 

The inhabitants mostly used ladders to descend the levels and the tunnels were used for the animals. 

There are at least 15,000 ventilation ducts in Derinkuyu that provide the residents with fresh air even when they are deep within the city. 

The large 55-meter (180-foot) ventilation shaft was also used as a well to provide water for those both above and below. Not all floors have wells that reach the surface to avoid being poisoned by the enemies during raids. 

Each level is connected to the next level by a hallway with a round stone door. The passages are narrow to force people to pass in a single file, making it easy to defend if enemies entered the city. 

The city also has a very spacious chamber with a barrel-vaulted ceiling, which is unique to Derinkuyu, on the second floor. It is believed that the chamber was used as a religious school and the rooms to the left were for studies. 

From the third and fourth floor, the way down is via vertical staircases that lead to a cruciform church on the lowest floor. 

Family rooms and communal places where people could meet, worship or work are connected by an extensive network of tunnels, passages, inclined corridors, and staircases. 

Derinkuyu also provided the residents protection from the weather which is very hot in summers and very cold in winters. 

The temperature in the city was stable throughout the year at 12°C (55°F) which is ideal for keeping the animals, maintaining fresh water supplies, and more importantly for keeping the food fresh. 

While there are as many as 18 floors in the city, only eight are accessible now. 

The entire underground city, as well as some of the houses in the town above, are carved from soft rock made from layers and layers of ash called “tuft” that spewed from the volcanoes millions of years ago.

As many as 200 underground cities, around 40 of them with three or more floors and the rest with a minimum of two floors, were discovered between Nevsehir and Kayseri.

Most of these underground cities and structures that were carved out of unique, geological formations are now archaeological tourist attraction sites.