This is the "Cistern of Theodosius" in Istanbul, built by Roman Emperor Theodosius II in 443 CE to serve the growing need for water in the capital of Constantinople, population: ~500,000
Its location was lost in the 18th century, and only recently rediscovered, restored, and opened in 2018.
This is the Basilica Cistern, built in the 6th century by Justinian I to provide constant water pressure to Constantinople's Great Palace and fountains district.
Some of the supporting columns have a unique feature.
Inverted medusa heads. It's unclear if these were repurposed stone taken from other locations (spoliation) or if they were a totem to guard against attacks by Gorgons by inverting the image.
Regardless, encountering them while exploring the cistern by torchlight would have been quite exciting?
It lay under Istanbul for centuries until some water board official noticed certain houses were pulling up water as if they had a well under their houses. And they did, it was the great Roman cistern fed from outside the city by Roman engineering of aqueducts & this magnificent cistern !!
Probably because being cisterns, they're underground. If you lose track of the entrance(s), people wouldn't know it's down there. My guess.
The Theodosius Cistern is 45m by 25m and the columns 9m high. (148ft x 82ft x 30ft)
