There are about 30 underground towns spread around the area of Nevsehir in Cappadocia, the largest is Derinkuyu which has six levels underground and could shelter about 20,000 people with livestock and food. These towns are connected to each other via underground tunnels, tunnel from Derinkuyu to Kaymakli is 8 km. They were built around 3,500 years ago by the Phrygians and became a stronghold of the Hittite empire. Watch 'Tribe from the depths of the Earth' on Youtube.