Vast underground cistern built by Justinian in 532 to store the city's water, the largest and best-known of the hundreds constructed. It's a cathedral-like space with 336 richly decorated columns. Time was you explored in a little rowing boat, but tourist numbers have soared and water levels have dropped, so now you follow a boardwalk, with lights, piped music and art installations. It's sometimes a film location, and stages concerts. But the crowds rob it of atmosphere and you might prefer to seek out one of the less-visited cisterns.

The Basilica Cistern, or Cisterna Basilica (Greek: Βασιλική Κινστέρνα, Turkish: Yerebatan Sarnıcı or Yerebatan Sarayı, "Subterranean Cistern" or "Subterranean Palace"), is the largest of several hundred ancient cisterns that lie beneath the city of Istanbul, Turkey. The cistern, located 150 metres (490 ft) southwest of the Hagia Sophia on the historical peninsula of Sarayburnu, was built in the 6th century during the reign of Byzantine emperor Justinian I. Today, it is kept with little water for public access inside the space.