The Basilica Cistern

Constantinople had a water problem that its founder does not seem to have anticipated. The site is superb for defence and trade and has almost no fresh water of its own: no river, few springs, and a population that grew within a century to several hundred thousand. Everything had to be brought in from the Thracian hills, and once brought in, stored against siege and summer.

The solution was one of the largest hydraulic systems built anywhere before the modern period — aqueducts running more than two hundred and fifty kilometres into Thrace, and a network of open reservoirs and roofed cisterns beneath the city. The Basilica Cistern is the largest of the roofed ones, and it is the only part of the system that most visitors ever see.

Construction

The cistern was built, or rebuilt on a larger scale, under Justinian I after the Nika riots of 532, as part of the reconstruction of the city centre that also produced Hagia Sophia.

It takes its name from the Stoa Basilica, a colonnaded public square that stood above it, near the Augustaion and the entrance to the Great Palace. A cistern had existed under the square since the fourth century; the sixth-century work enlarged it enormously.

The dimensions are the substance of the thing:

Area	About 138 × 65 m (roughly 9,800 m²)
Columns	336, in 12 rows of 28
Column spacing	About 4.9 m
Height	About 9 m to the vault
Capacity	Around 80,000 cubic metres

The columns carry brick groin vaults. The walls are roughly four metres thick, faced with a waterproof mortar containing crushed brick — the standard Roman hydraulic render. Procopius, in his account of Justinian’s building works, mentions the cistern; the construction is said to have used several thousand labourers, many of them enslaved.

Spolia

Almost all of the 336 columns are reused from earlier buildings, and it shows. They vary in height, diameter, material and order — Corinthian predominates, with some Doric and some plain shafts — and the shorter ones sit on higher plinths to bring the capitals to a common level.

This is not laziness. Reuse of architectural material (spolia) was normal Roman and Byzantine practice, driven by cost, availability and the speed of the building programme: quarrying and carving 336 new columns would have taken years, while dismantling redundant colonnades took months. The practice is discussed further in Byzantine sculpture.

One column, in the north-west section, is carved with a pattern of eyes and tears — usually called the “hen’s eye” or “peacock’s eye” column — and is traditionally said to commemorate the workers who died in the construction. The tradition is late and unverifiable; the carving is genuine and probably came from a triumphal monument.

The Medusa Heads

Two column bases in the north-west corner are carved Medusa heads: one lying on its side, one upside down. They are the most photographed objects in the cistern and the subject of a great deal of invented explanation — that the orientation was intended to neutralise the gorgon’s petrifying gaze, that it expressed Christian contempt for paganism, and so on.

The likeliest explanation is duller and better supported: they are Roman blocks of the second or third century, reused as bases, and they were placed in whatever orientation brought the column to the right height. Their original context is unknown.

How the Water Got There

The cistern was fed by the aqueduct system that reached the city from the Belgrade Forest and, at its furthest, from springs near Vize in Thrace. The best-known surviving element is the Aqueduct of Valens, completed in 373, whose double-arcaded span still crosses a main road in Istanbul — about 900 metres of it survive out of an original kilometre.

The full system, mapped in detail since the 1990s, includes channels, bridges and tunnels totalling more than 400 kilometres, making it the longest water supply line of the ancient world. Water arriving in the city was distributed to open reservoirs (three enormous ones survive as sunken parks and a football pitch), to roofed cisterns — over a hundred are known beneath modern Istanbul — and to public fountains and baths, described in Byzantine baths.

The strategic logic was siege. A city with the Theodosian walls and eighty thousand cubic metres of stored water under its main square could not be thirsted out, and no besieger ever managed it. Cutting the aqueducts, which the Avars did in 626 and others attempted later, was a serious blow but never decisive.

Rediscovery

The cistern was in use into the Ottoman period but was largely forgotten as a monument. The French scholar Petrus Gyllius, in Constantinople in the 1540s researching antiquities, noticed that residents of the houses above were drawing water — and catching fish — through holes in their basement floors. He had himself rowed through the columns by torchlight and published the description, which is the reason the cistern is known to modern scholarship.

The Turkish name, Yerebatan Sarnıcı, means “the sunken cistern”; it is also called Yerebatan Sarayı, “the sunken palace,” which is a fair description of the effect.

Visiting

The cistern was cleaned and opened to visitors in 1987, with raised walkways, and closed for a major restoration between 2017 and 2022. It reopened with new lighting, a sound installation and contemporary sculpture placed among the columns — changes that have been received with enthusiasm by some visitors and irritation by others who preferred the dripping, dim, fish-haunted version.

It is directly opposite Hagia Sophia and takes half an hour. The floor is damp and uneven, the light is deliberately low, and the acoustic — water dripping into shallow pools under a nine-metre brick vault — is the thing worth stopping for. Whatever one makes of the lighting design, the engineering is unaltered: a sixth-century vaulted hall the size of two football pitches, still holding water, under a city that has been rebuilt above it four times.

The Aqueduct System

The cistern is the visible end of one of the largest engineering works of the ancient world, and it is worth putting the numbers on it.

The supply reached Constantinople from Thrace by two main lines. The shorter, the Hadrianic system, drew from springs in the Belgrade Forest about twenty kilometres out. The longer, developed under Valens and extended into the fifth century, ran from the Vize region some 120 kilometres away in a straight line and roughly 250 kilometres along its course.

Survey work since the 1990s, principally by Kâzım Çeçen and by James Crow’s team, has mapped more than 400 kilometres of channels, tunnels and bridges — the longest water supply line known from antiquity, exceeding anything at Rome.

The channels are mostly covered conduits, with masonry bridges carrying them over valleys. More than sixty bridges have been identified, several still standing to considerable height in forest. The gradient is gentle and consistent, which for a line of that length implies survey work of a high order.

Inside the city the water fed three enormous open reservoirs — the cisterns of Aetius, Aspar and St Mocius, each several hundred metres across, now occupied by a park, a sports ground and gardens — and over a hundred covered cisterns, of which the Basilica is the largest.

Not the Only One

Visitors generally see the Basilica Cistern and assume it is unique. It is the biggest, and it is one of many.

The Cistern of Philoxenos (Binbirdirek, “the thousand and one columns”) lies a few hundred metres away and is the second largest covered cistern: 224 columns in a hall about 64 by 56 metres, with the unusual feature that its columns are made of two drums joined by a stone collar. It has been open to visitors intermittently and used for events.

Dozens of smaller cisterns survive beneath houses, mosques and shops across the historic peninsula, many known only from brief archaeological notices, some still holding water. Several are accessible through carpet shops and restaurants in Sultanahmet, which is an unusual way to encounter sixth-century vaulting.

Anyone with an interest in the subject should also look at the Cistern of Aetius, now the Vefa Stadium — a football pitch sitting inside a fifth-century reservoir whose retaining walls are the stands.

The 2017–2022 Restoration

The cistern closed for five years for structural work and reopened in 2022 substantially changed, which has divided opinion.

The engineering was necessary: columns had shifted, the vaults needed consolidation, and the Marmara earthquake risk required attention. Steel reinforcement was introduced and the walkways rebuilt.

The presentation changed at the same time. New lighting replaced the dim red-orange scheme of the 1987 arrangement; a sound installation was added; and contemporary sculptures were placed among the columns.

Reactions have been mixed and the disagreement is genuine rather than merely nostalgic. The older arrangement — very dark, dripping, with carp visible in shallow water and almost no interpretation — conveyed the experience of a working cistern. The newer one is better lit, more accessible and more legible, and reads more like a venue.

The sixth-century structure is unaltered either way, and the acoustic — water falling into shallow pools under a nine-metre brick vault — survives the redecoration.

• The Reign of Justinian I — the building programme it belongs to
• Byzantine Sculpture — capitals, spolia and reuse
• Daily Life in Constantinople — where the water went
• The Walls of Constantinople — the other half of siege-proofing
• The Great Palace — the district above the cistern
• Byzantine Science and Technology — the engineering tradition
• Constantinople to Istanbul — the city over the cistern since 1453