It might seem that everything erected in Constantinople in the 5th and the 6th centuries was of colossal proportions: mainly regarding its public buildings and fortifications, but also the water supply system. The city, founded in the prominent and admirable location and surrounded by water lacked one crucial resource - a local supply of fresh water. Wells and small water sources at first met this need, but soon they could not meet the growing needs of the city's population.

The problem had been older than Constantinople, as the first conduit system to bring water from afar, 47 km long, was created by Emperor Hadrian in the 2nd century CE, when the settlement had been known as Byzantium. This solution soon became insufficient for the growing city and was greatly extended. When Emperor Constantine proclaimed the city to be the new capital of the Roman Empire in 330 CE, the population grew rapidly. Thus, Constantine not only built monuments to decorate the city, but also started the construction of the longest water conveyance system in the Roman Empire. It was further extended in the second half of the 4th century by Emperor Valens. The fragment of this system still can be seen today in the centre of Istanbul - as the famous Valens Aqueduct, known in Turkish as Bozdoğan Kemeri, meaning the Aqueduct of the Grey Falcon.

Valens Aqueduct originally brought water from the slopes of the hills between Kağıthane and the Sea of Marmara. However, it was merely one of the terminal points of the new wide water-conveyance system, further extended in the 5th century. This system, that allowed the New Rome to flourish and develop, was in keeping with the great wealth and political importance of the city. The total length of its channels was certainly over 400 km long, and but possibly even longer, reaching 564 km. This system brought fresh water from the springs 120 km away from Constantinople as the crow flies. The water was coming from the springs deep in Belgrad Forest, located to the north-west of the city, as well as other locations in the Thracian hinterland. The city's system consisted of more than 60 aqueduct bridges as well as numerous tunnels, some of them reaching 1.5 km in length. The water was distributed to numerous public buildings and fountains of Constantinople, including the Great Palace and the Baths of Zeuxippus where Haseki Hürrem Sultan Baths were later erected by Mimar Sinan.

The water-conveyance system consisted not only of aqueduct bridges and tunnels, but also of division tanks, pressure-relieving towers, inverted siphons, and lead pipes. Moreover, the storage of enormous amounts of water delivered to Constantinople is the most distinctive feature of its water-conveyance system. In this city, the cisterns formed a unique storage and distribution system, far more extensive and developed than in any other location. It also meant that the system required significant regular maintenance and management.

The total number of the cisterns that existed in Constantinople remains a question of scholarly debate. Petrus Gyllius, a French natural scientist, topographer and translator, spent the years 1544 to 1547 in Constantinople. He had been there sent by King Francis I of France in order to find ancient manuscripts. Among other discoveries, he described nine ancient cisterns, some still in use, during his time in the city.

The recent research has increased the number of the known cisterns to almost unbelievable amounts. In the latest studies, there are 211 Byzantine-era cisterns catalogued, and this astonishing quantity signifies their crucial importance for Constantinople’s water supply strategy. Some of them are huge unroofed reservoirs, such as Cistern of Aetius, Cistern of Aspar, and Cistern of Mocius. They were situated on the periphery of the city, between the Constantinian and Theodosian Walls, where population density was very low and space plentiful. The three main reservoirs could hold more than 600 thousand cubic meters, that is three-quarters of the known storage volume within Constantinople. They may have supplied water for baths and agricultural needs, but it is also possible that they fed the rest of the system in the times of need.

Smaller, roofed cisterns and reservoirs, for instance, Basilica, Philoxenos, and Theodosius Cisterns provided drinking water. Almost all of them were fed by the water-conveyance systems rather than by rainwater as testified by their large dimensions. There are only fourteen cisterns discovered so far with the volume smaller than 100 cubic meters. The spatial arrangement of the cisterns reflected the density of the city's population, with the largest number around the first two hills of Constantinople. This was the oldest inhabited area of the city with the largest population and this is the location of all the large covered reservoirs.

This amazing system provided a secure water supply for the city for hundreds of years. According to the information provided by Codex Theodosianus, some households of Constantinople had their own water supply as the laws regulated the size of the supply pipes. However, the majority of the people had to rely on public fountains to get their drinking water. This need resulted in the regular and dense network of cisterns below the city that facilitated access to water.

The water conveyance system, crucial for the inhabitants of the city, underwent major renovations in the 6th, the 8th, and the 12th centuries. After the conquest of Constantinople by the Turks in 1453, it is estimated that the population was 50 thousand people, but it increased rapidly. The water conveyance systems required repairs and renovations, and they were carried out during the reign of Sultan Mehmed II. However, as the population of the city skyrocketed during the reign of Sultan Suleyman the Magnificent and reached 175 thousand in the mid-16th century, new solutions became urgently needed. The solution was the most famous Ottoman architect, Mimar Sinan, was given the task to create a new water supply system. He used some elements of the ancient lines but greatly expanded them. In his biography, Sinan described the aqueducts he had constructed in the area of Belgrad Forest to provide fresh water for the inhabitants of Istanbul in such great detail that it seems they were more significant to him than the great mosques such as Şehzade or Süleymaniye.

Among numerous remains and traces of Constantinople's water supply system, Basilica Cistern is certainly best-known. However, it is not the only cistern preserved in the historical centre of the city. The Theodosius Cistern is located around half a kilometre from Basilica Cistern, but remains rather unknown and rarely sees crowds of visitors. The cistern was built in the 5th century to store water supplied by the Valens Aqueduct. It was not the first such structure in the city, as the Philoxenos (Binbirdirek) Cistern had been built much earlier, in the 4th century. It was not the last one, either, with the most famous Basilica Cistern added in the 6th century.

The name of the Theodosius Cistern comes from Emperor Theodosius II who ruled the Eastern Roman Empire in the first half of the 5th century. He is best-remembered for promulgating the Theodosian law code and for the construction of the Theodosian Walls of Constantinople. Another building erected during his reign was the second Hagia Sophia - the intermediary phase between the Great Church inaugurated in 360 during the reign of Constantius II and the third Hagia Sophia erected by Justinian in 532-537.

The second church, built in the times of Theodosius II, replaced the Great Church that had been burned down during the riots that broke out in 404. They resulted from the conflict between the Patriarch of Constantinople, John Chrysostom, and Empress Aelia Eudoxia, the wife of the emperor Arcadius - the father and co-emperor with Theodosius II. The second church on the site was ordered by Theodosius II, and it was inaugurated in 415. It was the basilica with a wooden roof, built by architect Rufinus. Out of this second church, that perished during the tumult of the Nika Revolt in 532, several marble blocks have been preserved, including the ones with the reliefs depicting 12 lambs representing the 12 apostles. These precious pieces were discovered in 1935 beneath the western courtyard of Hagia Sophia by the German archaeologist Alfons Maria Schneider. However, further digging was abandoned because of the threat to the integrity of the building. They are now on display near the entrance to Hagia Sophia.

Because of these ambitious construction programmes, also the creation of the cistern was attributed to Theodosius II, as a part of the water supply reorganisation programme. Unfortunately, there are no historical sources that could confirm the connection of this structure with this emperor. Moreover, according to Notitia Urbis Constantinopolitanae, a list of monuments, public buildings and civil officials in Constantinople during the mid-5th century, the Cistern of Theodosius was located within Region V, at the opposite side of the Mese, the main thoroughfare of ancient Constantinople. On the other hand, the cistern discussed here was mentioned by Marcellinus Comes, a Latin chronicler of the Eastern Roman Empire writing in the 6th century as Cisterna Maxima. However, the misleading name - Theodosius Cistern - is still commonly used, even in official tourist information provided by the municipality of Istanbul. The Turkish name of the cistern - Şerefiye Sarnıcı - means Goodwill Cistern.

The rectangular Theodosius Cistern is rather small, especially in comparison to the grand Basilica Cistern (138 metres by 65 metres), as it is only about 42 meters long and 25 meters wide. Its vaulted ceiling is made of red bricks and supported by four rows of eight nine-meter-high marble columns with Corinthian capitals. The walls were also erected of bricks, around 35 cm long and 5 cm thick, separated by thick joints.

Although these dimensions make it smaller than the Basilica and Binbirdirek cisterns, the visit to the Theodosius Cistern is a very rewarding experience. On the one hand, it is never as crowded as the best-known structure of this kind in Istanbul - the Basilica Cistern. On the other hand, it has a much more pleasant ambience and offers a more exceptional esthetic experience than the Binbirdirek Cistern as it is filled with water, similarly to the Basilica Cistern. Thus, it combines the best features of the other two cisterns: magical water-world atmosphere and splendid solitude.

This cistern is also a relatively new point on the map of Istanbul's tourist attractions as it was open for tourists in April 2018. The cistern was discovered by pure luck in 2010. Back then, the Istanbul Metropolitan Municipality decided to demolish the building of Eminönü Town Hall. Surprisingly, the works revealed the existence of the previously unknown cistern hidden beneath it. Understanding the great significance of this discovery, the authorities quickly initiated renovation works to prepare the cistern for visitors. Today, the cistern is located below a modern building erected by the architect Cafer Bozkurt mainly of glass and steel, with wooden floor. This architectural solution creates a very intriguing contrast between the ancient structure below and the modernistic construction above it. The cistern is not only a tourist attraction as it also hosts art exhibitions and cultural events.

This text is a fragment of a guidebook to Istanbul: "Byzantine Secrets of Istanbul".