Built by the Emperor Justinian at the same time as the Haghia Sophia, it was forgotten for centuries and only rediscovered by a Frenchman, Peter Gyllius, in 1545 when he noticed that people in the neighbourhood got water by lowering buckets through holes in their basements.
It's a tremendous engineering feat, with brick vaults supported on 336 columns spaced at four-metre (13-foot) intervals. Prior to restoration in 1987, the cistern could only be explored by boat (James Bond rowed through in From Russia With Love). These days there are concrete walkways. The subdued lighting and subterranean cool are especially welcome on hot days.
Look for the two Medusa heads at the far end from the entrance, both recycled from an even more ancient building and casually employed as column bases. There's a café down here and a platform on which occasional concerts of classical Turkish and Western music are performed; check with the ticket office for further details.