Cavernous caves of the surviving the Byzantine-era (and the unearthed bodies)!
The Basilica Cisterns (330–1453AD) can be found atmospherically lit in the area of Sultanahmet, arguably the most historic part of the city. The subterranean maze was built in 532 and forms the largest of the surviving Byzantine cistern in İstanbul; featuring some 336 columns, many these salvaged from ruined temples.
It first lay underneath one of the great squares and was designed to service the Great Palace and was able to store 80,000 cubic metres (800 litres) of water delivered using 20km of aqueducts from a reservoir near the Black Sea. When emperors relocated from the Great Palace, it was forgotten about until 1545, when locals were spotted lowering buckets into the ground and pulling out fish. Then came the Ottomans, who weren’t too keen on preserving it, and instead used it to dump rubbish and even the odd body or two down there.
It was finally cleaned-up by authorities in 1985, renovated and opened to the public in 1987. Now you can walk on wooden platforms and watch carp swim past.
Other underground worthies include the brick vaults of Sultan Sarnıçı’s brick vaults which host trendy events each weekend.