Basilica Cistern

Down a small stairway from a busy street, the noise fades and the air turns cool and damp. You emerge onto a walkway above dark water, into a vast underground hall held up by a forest of stone columns vanishing into shadow. This is the Basilica Cistern, a Byzantine reservoir hidden beneath the old city.

It was built in the sixth century, under the emperor Justinian, to store water for the great palace and buildings of the Byzantine capital above. Water was carried here by aqueduct from far outside the city and held in this enormous tank against siege and drought.

The cistern is huge, a space the size of a cathedral set underground, its brick-vaulted roof carried on hundreds of columns in long rows. The name comes from a great basilica that once stood above it; the reservoir took its place in the city's memory once the building was gone.

Many of the columns were reused from older Roman buildings, so they vary in style and stone, a quiet record of recycling on a grand scale. One famous column is carved all over with the shapes of teardrops, and stories have grown up to explain it.

The strangest sight lies in a far corner, where two great carved heads of Medusa, a monster from Greek myth, serve as bases for two columns. One is set on its side and the other upside down, and no one is quite sure why, which has fed centuries of speculation.

The cistern is above all an experience of mood. Dim light, still water, the slow drip of moisture, and soft music make it one of the most atmospheric spaces in the city, a complete change from the bright, crowded streets overhead.

It has been cleaned, drained, and restored over the years, with raised walkways now letting visitors move dry-shod above the shallow water, where fish drift between the columns. Lighting picks out the rows of pillars and throws long reflections across the surface.

For a long time the cistern was half-forgotten, and people in the houses above were said to draw water, and even fish, through holes in their floors without knowing what lay beneath. Its rediscovery and study came only later, adding to its air of secrecy.

In recent years the restored space has also been used for art installations and events, and sculptures sometimes stand among the columns. The combination of ancient engineering and modern art gives the place an unexpected, dreamlike quality.

The cistern sits in the very heart of the old city, a short walk from Hagia Sophia and the great mosques, so it makes an easy and refreshing stop, especially welcome as a cool refuge on a hot day.

A few practical notes. Entry is ticketed and the cistern is now very popular, so queues form; booking ahead or coming early helps. The walkways can be slippery and the light is low, so step carefully, and bring a layer, as it is cool below.

Pause at the far end by the Medusa heads, with the columns marching away into the dark and the water still beneath you. This silent hall kept a great city alive for centuries, hidden under its feet. When you climb back up into the noise and light of the street, it is hard to believe what lies just below.