Basilica Cistern

Step through a small unassuming door on Yerebatan Caddesi and you descend into a subterranean basilica the size of a cathedral. Built in 532 CE by Emperor Justinian to supply water to the imperial palace and Topkapı, the Basilica Cistern (Yerebatan Sarnıcı) could hold 80,000 cubic meters of water delivered by aqueduct from the Belgrade Forest, 20 km away.

The 336 columns were mostly salvaged from Roman ruins across the empire — you can spot Corinthian and Doric capitals that don't quite match, recycled from older temples. Look for the two Medusa heads used as column bases in the northwest corner. Nobody knows why; the best guesses range from "purely structural" to "spolia of a Roman temple to Medusa." Either way, they're the most photographed stones in the cistern.

Wooden walkways thread between the columns about 30 cm above the water. The acoustics are extraordinary — a single dropped coin echoes for seconds. Allow 30-45 minutes. Combine with Hagia Sophia across the square.