Derinkuyu Underground City
"Derinkuyu Underground City is an ancient multi-level underground city of the Median Empire in the Derinkuyu district in Nevşehir Province, Turkey. 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 is the largest excavated underground city in Turkey and is one of several underground complexes found across Cappadocia.
It was opened to visitors in 1969 and to date, about half of the underground city is accessible to tourists." Wikipedia
They say it was built/ dug in the 6th or 7th century BC. That's a long time ago for structures that are so advanced. Only 10% of it is open to the public and a lot of the rest isn't even excavated.
"First built in the soft volcanic rock of the Cappadocia region, possibly by the Phrygians in the 8th–7th centuries B.C. according to the Turkish Department of Culture, the underground city at Derinkuyu may have been enlarged in the Byzantine era. During the Persian Achaemenid empire the city was used as a refugee settlement. There are references to underground refugee settlements built by the Persian king Yima in the second chapter of the Zoroastrian bookVendidad. Therefore many scholars believe that the city may have been built by the Persians.The city was connected with other underground cities through miles of tunnels.
Some artifacts discovered in these underground settlements belong to the Middle Byzantine Period, between the 5th and the 10th centuries A.D." Wikipedia