This blows my mind, purely because the tools and manpower needed to make these ancient cities underground is immense. Not only are some of the cities complex, but they are also stable and navigated by a system of tunnels. Here is one of the maddest underground city.
One of the biggest underground cities in the world is Derinkuyu, it forms part of the massive spread of Cappadocia in modern day Turkey. The system of tunnels digs down about 60 metres and could accommodate approximately 20,000 people!
The soft volcanic rock made it easier to dig through, and was probably built by the Phyrgians in the 8th-7th century BC. It was expanded in the Byzantine era, and was later ruled by the Persians.
The crazy tunnels and stability of the place led to it being a refuge for religious reasons all the way up to the 10th century AD.
There are many underground cities around the world, some are pretty recent, being built for reasons like secret passageways, subways and more. All in all the ancient ones are the most impressive purely because they didn’t have bulldozers, and have managed to stand the test of time and still be safe to walk through today.