What technology did ancient civilisations have that we still don't understand today?

 Turkey's underground city of 20,000 people-

More than 85m beneath the famous fairy chimneys of Cappadocia lies a massive subterranean city that was in near-constant use for thousands of years.

The ancient city of Elengubu, known today as Derinkuyu, burrows more than 85m below the Earth's surface, encompassing 18 levels of tunnels. The largest excavated underground city in the world, it was in near-constant use for thousands of years, changing hands from the Phrygians to the Persians to the Christians of the Byzantine Era. It was finally abandoned in the 1920s by the Cappadocian Greeks when they faced defeat during the Greco-Turkish war and fled abruptly en masse to Greece. Not only do its cave-like rooms stretch on for hundreds of miles, but it's thought the more than 200 small, separate underground cities that have also been discovered in the region may be connected to these tunnels, creating a massive subterranean network.