The Basilica Cistern: Istanbul’s Underground Byzantine Reservoir

A short walk from Hagia Sophia, a plain entrance on Yerebatan Caddesi leads down a flight of stone steps into the largest of the city’s surviving covered Byzantine cisterns. The Basilica Cistern was never meant to be seen: it is a 6th-century reservoir, a hall of 336 columns standing in water under brick vaults, built to keep the imperial city supplied through the dry summers. Known in Turkish as Yerebatan Sarnıcı, the “sunken cistern”, and popularly as Yerebatan Sarayı, the “Sunken Palace”, it is now one of Istanbul’s most visited monuments, and it was substantially restored and re-lit between 2017 and 2022. On Day 3 of our Small Group Tour of Ancient Turkey, the group descends into the cistern at the end of a day that begins at Topkapi Palace.

Why Justinian built the basilica cistern

Constantinople had a chronic water problem. The peninsula had few springs of its own, so the city depended on aqueducts bringing water from the hills and forests to the north, and on storage to see it through summer. Under Justinian I (r. 527–565), that storage was expanded on an imperial scale. The date usually given for the Basilica Cistern is 532, the year the Nika riot burned much of the monumental centre, though some scholars place the start of work a few years earlier. Justinian is also recorded as restoring the old Aqueduct of Hadrian in 528, which probably fed the cistern.

The cistern takes its name from the building above it. The Stoa Basilica was a large colonnaded court on the First Hill that housed law courts and was linked to a library; it had burned in 476 and burned again in 532. Justinian’s court historian Procopius, in his Buildings, describes the emperor excavating one of its stoas to create a reservoir for the summer months, and concludes: “Thus the Emperor Justinian made provision that the people of Byzantium should not be in want of fresh water.” For more on how the city grew around its imperial monuments, see our overview of Istanbul as an imperial capital.

How the cistern was built and supplied

The reservoir is a vast rectangle, roughly 140 metres long and 70 metres wide, enclosing close to 10,000 square metres. Its brick walls are four to nearly five metres thick, and the brick floor was sealed with a waterproof lime mortar known in Turkish as horasan, a mix of lime and crushed brick that the Byzantines used widely in water structures. Full, it held something like 80,000 cubic metres of water.

The roof is carried by 336 columns, each about nine metres tall, set in 12 rows of 28 at intervals of just under five metres. They support shallow brick cross vaults, which spread the load evenly across the grid. Water most likely arrived through the Aqueduct of Hadrian, though the city’s water network has changed so much since that the exact Byzantine route is uncertain. When the 2022 restoration cleared later concrete from the walls, it exposed two old supply pipes, one leading towards Hagia Sophia and one towards a palace building that predates Topkapi. Historians are still unsure whether the cistern served the public at large or chiefly the palace and church complex around it.

The 336 columns: a reused forest of stone

Look closely and the columns are anything but uniform. Most of the shafts and capitals appear to be spolia, material salvaged from older buildings, and they come in several marbles and granite. Most capitals are Ionic or Corinthian, with a few plain Doric examples. This was efficient building: Justinian’s architects needed hundreds of identical-height supports quickly, and the city’s ruined and redundant buildings supplied them.

One column stands out. Its shaft is carved with raised motifs that look like a hen’s eye, slanting branches and falling drops, and it is often called the “weeping column”. The pattern closely resembles columns from the late 4th-century Triumphal Arch of Theodosius I, which suggests where at least some of the stone came from.

The Medusa heads

In the northwest corner, two columns stand on large marble blocks carved with the head of Medusa, the Gorgon of Greek myth whose gaze turned onlookers to stone. One block lies on its side, the other upside down. Their origin is unknown: they were most likely taken from a late Roman building, possibly the Forum of Constantine, where a similar head has been found, and there is no evidence that they were carved to serve as column bases. Whether they were turned deliberately, to neutralise the Gorgon’s power, or simply set at whatever angle gave the right height, is an open question. The heads lay buried in silt until the 1985–1987 cleaning brought them to light.

Rediscovery, Ottoman repairs and the modern museum

After the Ottoman conquest of 1453 the cistern supplied Topkapi Palace for a time and later the public, but it slipped from the notice of the wider city. In the mid-16th century the French scholar Pierre Gilles (Petrus Gyllius), studying the topography of Constantinople, found that people living above it drew water through wells in their basements; he measured the cistern and was rowed among the columns, noting fish in the water below. The Ottomans repaired it in 1723 under Ahmed III and again under Abdulhamid II. Between 1985 and 1987 the Istanbul Metropolitan Municipality removed tens of thousands of tons of silt, laid walkways in place of boats, and opened it as a museum in September 1987.

What changed in the 2017–2022 restoration?

A great deal, most of it aimed at earthquake safety. Partially closed from 2017 and fully from 2020, the cistern reopened on 22 July 2022. The old iron ties linking the columns were replaced with stainless-steel rods, and the restorers removed around 1,440 cubic metres of 20th-century concrete, a thick later cement render on the walls and some 1,600 cubic metres of sediment, revealing the original 6th-century brick floor. The old concrete walkway gave way to a modular steel walkway set apart from the historic fabric and running within about half a metre of the water. New LED lighting makes the floor visible through the shallow water and shifts in colour across the hall, and the reopening added contemporary art installations among the columns.

How do you get down into the Basilica Cistern?

By stairs: visitors descend a flight of 52 steps from the street entrance. We could not confirm a step-free route at the time of writing, so anyone who needs one should ask before travelling. Inside, visitors follow raised walkways above the water, and the light is dim, so allow a moment for your eyes to adjust before moving on.

How long does a visit take, and what is it like underground?

Most visitors spend 45 minutes to an hour, which is ample to walk the full circuit and stop at the Medusa heads. The cistern stays noticeably cooler than the street even in midsummer, so a light layer is welcome, and surfaces near the water can be damp. Expect the busiest moments at the Medusa heads, where people pause for photographs.

Is the Basilica Cistern open, and who runs it now?

It is open daily at the time of writing, but arrangements have been in flux. In June 2026 management passed from the Istanbul Metropolitan Municipality to Turkey’s General Directorate of Foundations, the site closed for a few days in early June before reopening, and the transfer remains before the courts. Opening hours, ticketing and any evening sessions may change, so check the latest information before you go.

The Basilica Cistern on our Ancient Turkey tour

The cistern closes the third day in Istanbul. The day begins at Topkapi Palace, the seat of the Ottoman sultans, followed by time in the Grand Bazaar, and the group visits the cistern before returning to the hotel. It makes a natural sequel to the previous day, when the group visits the Hippodrome and Justinian’s great church of Hagia Sophia, built in the same years and by the same emperor. Seeing the two together shows the full reach of his building programme, from the soaring dome to the engineering that kept the city alive below it.

A local English-speaking guide accompanies the group alongside the Odyssey Program Leader. The tour is rated Level 2, Moderate: participants should be comfortable with 3–5 hours of physical activity a day, the equivalent of walking up to 8 kilometres on uneven ground, and with climbing and descending stairs. Our 21-day Ancient Turkey itinerary from Istanbul to Gallipoli sets these Istanbul days within the full journey through Anatolia. To join a small group of up to 16 travellers, see the Small Group Tour of Ancient Turkey.