Geology as architecture
Derinkuyu exists because of a unique geological formation in the Cappadocia region. Millions of years ago, volcanoes like Mount Erciyes and Mount Hasan blanketed the area in thick layers of volcanic ash. This ash compacted into a rock called tuff. Tuff is porous and soft, making it easy to carve with simple tools, but it hardens when exposed to air. This property allowed ancient peoples to excavate extensive subterranean structures without the need for complex machinery. The result is one of the largest and deepest excavated underground cities in the world, descending at least eight levels to a depth of 85 meters (279 feet). Though archaeologists suspect there are as many as 18 levels in total, none of the floors have ever collapsed, a sign of the stability of the tuff rock.
The initial construction of the city is often attributed to the Phrygians in the 8th-7th centuries BCE. The city was significantly expanded over centuries, particularly during the Byzantine era when early Christians used it as a refuge from persecution. The complex could shelter up to 20,000 people along with their livestock and food stores for extended periods.
An ecosystem engineered for survival
The design of Derinkuyu shows a sophisticated understanding of engineering and resource management. A central 55-meter (180-foot) ventilation shaft sent fresh air to the deepest levels and was also a primary well, supplying clean water. This main shaft is part of a larger network of over 50 smaller shafts that ensure air circulation throughout the complex.
The city was a self-contained world. Different levels had specialized functions: the upper floors housed stables and livestock, which kept the lower living quarters warmer and contained smells near the surface. Deeper levels contained living quarters, kitchens, storage rooms, wine and oil presses, chapels, and even a large barrel-vaulted room thought to have been a religious school. For defense, each level could be sealed off from the inside with massive, rolling stone doors. These circular doors, weighing up to 500 kg, were impossible to move from the outside once rolled into place across a corridor. Long tunnels, some stretching for kilometers, are believed to have connected Derinkuyu to other underground cities in the region, such as Kaymakli.