Hidden beneath the streets of Istanbul lies one of the most extraordinary surviving structures of the Byzantine world. Built in the sixth century during the reign of Emperor Justinian I, Yerebatan Sarnıcı – the Basilica Cistern once formed part of the vast hydraulic network that supplied water to Constantinople. Today, its endless rows of columns, shallow reflective water and shifting underground light create one of the most atmospheric spaces anywhere in Europe.

Yerebatan Sarnıcı – the Basilica Cistern | Yerebatan-Zisterne – die Basilika-Zisterne | Yerebatan Sarnıcı – la Citerne Basilique | Yerebatan Sarnıcı – Cysterna Bazyliki | Yerebatan Sarnıcı – la Cisterna Basílica | 地下水宫——巴西利卡地下蓄水池 | イェレバタン地下宮殿－バシリカ・シスタン | 예레바탄 사라이 – 바실리카 시스턴

This will probably be my last post about Istanbul for a while. Another trip is already slowly approaching and more folders are waiting to be sorted through. Still this post is about one of the most extraordinary places to see not only in Istanbul, but possibly also on the European scale.

One scene from Inferno stayed with me long after I first watched the film. There is a moment inside Hagia Sophia when the protagonist presses his ear against the floor and realises there is water somewhere beneath him. The scene creates a very clear impression that the enormous underground cistern lies directly below the building itself. For a long time, that was exactly how I imagined it. Only once I arrived in Istanbul did I realise that it is not exactly true. Yerebatan Sarnıcı – the Basilica Cistern is not actually located directly underneath Hagia Sophia. It stands a short distance away. Above ground, everything feels entirely contemporary – traffic, crowds, cafés, tour groups and city noise. Yet only a few metres below street level lies a vast piece of Byzantine infrastructure that has survived for nearly fifteen hundred years.

At the same time, the film was not completely wrong. This entire part of ancient Constantinople is threaded with underground structures – cisterns, tunnels, channels and forgotten hydraulic systems built to collect, store and distribute water throughout the city. Even beneath Hagia Sophia itself there are underground chambers and water-related structures dating back centuries. So although the geography was simplified for cinematic effect, the film captured something real – the feeling that there is another hidden city beneath the streets of Istanbul.

You enter the cistern by descending a staircase. Above ground there is the ordinary rhythm of the city – traffic, voices, the sound of buses passing nearby – and then, step by step, you move down into darkness, cool air and silence broken only by echoes. It feels as though you are entering a vast hidden structure concealed beneath the city itself.

My first impression after entering was simply the scale of it all. Of course I had seen photographs beforehand, and I knew the location from the film, but images do not really prepare you for the actual size of the space. On screen it looks like an atmospheric underground hall. In reality, it feels far larger – more like entering an entire subterranean world than a single architectural space. And almost immediately, the cinematic version collides with reality. In Inferno, the cistern appears almost empty, silent and permanently drenched in deep red light. In real life, it is one of Istanbul’s most visited attractions, and there are people everywhere. Tourists are constantly taking photographs, conversations echo beneath the vaulted ceiling, footsteps bounce off the stone surfaces, and every now and then someone nearby uses flash photography. Rather than an abandoned underground chamber, it feels alive, busy and intensely visited.

To be honest, one of the first things I found myself doing was trying to work out how to photograph the space while avoiding people in the frame. Places like this are always a challenge, especially when the lighting changes constantly and somebody nearby suddenly fires a flash at exactly the wrong moment. Yet that constantly shifting illumination is also part of what gives the cistern its atmosphere. There are moments when the entire interior glows red, just as it does in the film. Then, only seconds later, the lighting shifts to green or blue. Spotlights positioned throughout the cistern – some low near the water, others higher among the columns – continually alter the mood of the space. Certain sections disappear into shadow before suddenly becoming illuminated again. Occasionally the lighting turns brighter and more neutral, allowing you to properly study the architecture itself.

And that is when you begin to notice what probably leaves the strongest impression of all – the columns. There are so many of them that eventually you stop trying to count. Looking ahead, it genuinely feels as though the forest of columns continues endlessly into the darkness. Reflections in the shallow water make the perspective feel even deeper, with rows of columns fading gradually into shadow. It is something that photographs capture surprisingly well. In several of my own images, the perspective seems almost endless, the lines of columns disappearing far beyond the visible space.

The water itself also looks very different from what many people might expect after watching the film. Today, only a relatively shallow layer remains across the floor of the cistern. But if you look carefully at the walls and columns, you can clearly see marks left by centuries of water erosion much higher up. It becomes obvious that the entire structure was once filled almost to the ceiling. And that is the moment when the scale of the undertaking truly begins to sink in. This was not a decorative underground chamber or a hidden cellar beneath a building. It was a gigantic piece of urban infrastructure designed to supply water to one of the greatest cities of the ancient world.

That was probably the aspect that fascinated me most. More than the mystery of the place, I found myself thinking about the engineering behind it. Of course the cistern is visually spectacular, and it is easy to understand why filmmakers are drawn to it. But standing among those endless rows of columns, the real questions become entirely different. Where did all this water come from? How was it transported here? How was it distributed throughout the city? What sort of system was required to keep Constantinople functioning?

The history of the Basilica Cistern is, in many ways, the history of Constantinople itself – a city that could never have survived without an enormous and highly sophisticated water system. Modern Istanbul is surrounded by water and closely associated with the Bosphorus, yet paradoxically the city struggled for centuries with access to reliable fresh drinking water. Constantinople was built across a series of hills, and its local water sources were far from sufficient for a metropolis that, at its height, may have approached a population of one million people. On top of that, the city lived under the constant threat of sieges. The Byzantines therefore needed a system capable not only of transporting water into the city, but also of storing vast quantities of it in case of crisis.

It was within this context that the Basilica Cistern was constructed. In Turkish, it is known as Yerebatan Sarnıcı, often translated as the Sunken Palace or Underground Palace – a surprisingly accurate name, because the interior feels far more like a monumental subterranean hall than a purely technical structure.

Construction began during the reign of Emperor Justinian I in the sixth century, most likely around 532 AD, shortly after the devastating Nika riots that destroyed large sections of Constantinople. Nika was the chant shouted by crowds during chariot races at the Hippodrome of Constantinople. At the time, the chariot racing factions were far more than sports supporters. They held enormous political, social and even religious influence within the city. Rivalries between them frequently turned violent. Growing public anger over taxation and Justinian’s policies eventually merged with tensions between the factions, leading to a massive revolt against the emperor himself. For several days, Constantinople was effectively in chaos. Crowds set fire to public buildings, street fighting spread across the city, and large parts of the imperial centre were destroyed. The earlier version of Hagia Sophia was among the buildings that burned during the riots. According to historical accounts, Justinian even considered fleeing the city. In the end, however, he chose to crush the uprising with extreme force. Imperial troops trapped large numbers of rebels inside the Hippodrome and carried out a massacre. Paradoxically, the destruction caused by the riots became the starting point for one of the greatest rebuilding programmes in Byzantine history. Justinian used the devastation as an opportunity to reconstruct Constantinople on an even more monumental scale. The present Hagia Sophia was built shortly afterwards, and the Basilica Cistern was also completed or significantly expanded as part of the city’s renewed infrastructure system.

The Basilica Cistern interior. Hidden between the columns are two enormous stone heads of Medusa, placed beneath supporting columns during the construction of the reservoir. One lies sideways, the other upside down, and no one knows with certainty why they were positioned this way. Some historians suggest the heads were deliberately inverted to symbolically neutralise the power traditionally associated with Medusa in the ancient world.

The cistern was designed as a gigantic underground reservoir supplying water to the Great Palace of Constantinople and to the most important buildings in the imperial centre of the city. Even today, its scale is extraordinary. The structure measures roughly 138 metres in length and 65 metres in width, covering nearly 10,000 square metres in total. In practical terms, it is an underground space comparable in size to several modern industrial halls hidden beneath the streets of Istanbul.

Its most distinctive feature is, of course, the forest of columns. There are exactly 336 of them, arranged in twelve rows containing twenty-eight columns each. Most stand approximately nine metres tall. Interestingly, many were not carved specifically for the cistern itself. The Byzantines frequently reused architectural elements from older Roman structures, temples and public buildings, which explains why not all of the columns look identical. Some feature different capitals and decorative details, giving the impression that the entire structure was assembled from fragments of several different historical worlds layered on top of one another.

The cistern itself was constructed primarily from brick and coated with a waterproof mortar designed to withstand constant exposure to water. In some areas, the walls are several metres thick, built to resist the immense pressure created by tens of thousands of tonnes of stored water. When completely full, the reservoir could hold around 80,000 cubic metres of water – tens of millions of litres in total. Standing inside the cistern today and looking up at the visible water marks left high on the walls, it becomes much easier to understand the sheer scale of the undertaking.

The water, of course, did not simply appear there naturally. The Basilica Cistern formed part of the vast hydraulic network that supplied Constantinople. Water was transported into the city through aqueducts stretching for many kilometres from forests, springs and reservoirs located west of the urban centre. The most famous surviving element of this system is the Aqueduct of Valens, parts of which can still be seen in Istanbul today. In practice, the entire network functioned as a huge interconnected system – aqueducts carried water into the city, which was then stored in reservoirs such as the Basilica Cistern before being distributed further throughout Constantinople.

And that is perhaps what makes the cistern so fascinating once you stop seeing it simply as a tourist attraction and begin viewing it as infrastructure. The Byzantines created a system capable of sustaining one of the largest cities in the medieval world for centuries. And they achieved this nearly fifteen hundred years ago, without modern pumps, computers or contemporary construction technologies.

Interestingly, after the Ottoman conquest of Constantinople, the cistern gradually lost much of its original importance. Ottoman residents tended to rely more heavily on flowing water and fountains than on vast underground reservoirs. For a period of time, the structure was even partially forgotten. Historical accounts describe local residents lowering buckets through openings in their floors to draw water directly from the cistern below, and in some cases even catching fish there.

European travellers only began rediscovering the site during the sixteenth century, when Western visitors started writing about the extraordinary underground reservoir hidden beneath the streets of Istanbul. Over time, the cistern was cleaned, restored and eventually opened to the public. Today it remains one of the city’s most famous landmarks, yet despite the crowds of tourists it still retains something of the atmosphere of old Constantinople – a city that depended on a hidden underground world of water, tunnels and enormous reservoirs in order to survive.