The site of Byzantion had a major shortage of water, and an extensive system of aquaducts and cisterns was created to maintain the water supply. Just across and down from Hagiah Sophia sits a lone, single-storey building that sits over the entrance to the Yerebatan Sarnıcı (Basilica Cistern), the grandest of several underground reservoirs that riddle the foundations of this part of the city.
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 4m (13ft) 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 cafe 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.
Open May-Oct 9am-7pm daily. Nov-Apr 9am-6pm daily. Admission $7. Credit MC, V.