It is an ancient multi-level underground city of the Median Empire in the Derinkuyu district in Nevşehir Province, Turkey (Cappadocia). Extending to a depth of approximately 60 m, it was large enough to shelter approximately 20,000 people together with their livestock and food stores.
It was opened to visitors in 1969 and to date, about half of the underground city is accessible to tourists.
Could there really be vast open spaces within the Earth where advanced civilisations live? If we were visited in the ancient past, could they have hidden the evidence deep within these tunnels? Perhaps we have only scratched the surface of our planets underground mysteries... and maybe there is more, much more to be found.