At some point in history, a highly developed civilization with its own culture, traditions, art, faith, laws, foundations - for some unknown reason went to live underground.
There are hundreds of underground cities on the territory of Byzantium (now Turkey). These cities have still not even been fully excavated and explored, scientists are still lost in guesses about the purpose of going underground and about the technologies with which they built it and with which they illuminated underground cities, how plants were grown there, where water was taken, and so on.
There are many hypotheses, there is no answer... Or maybe history repeats itself and we are being led to the same thing?