Basilica Cistern Fish: Why Are There Fish Underground?

Deep beneath the streets of Istanbul, the Basilica Cistern hides an unusual story. Beyond its 336 ancient columns, mysterious Medusa heads, and atmospheric reflections, this underground reservoir has a surprising connection with fish. For centuries, fish have been part of the history of the Basilica Cistern. Historical accounts describe people catching them beneath their homes, while some modern visitors have reported seeing fish swimming between the columns. But why are there fish in the Basilica Cistern, how did they get there, and can visitors still see them today? The answers reveal a lesser-known side of one of Istanbul's most fascinating Byzantine monuments.

Are There Fish in the Basilica Cistern Today?

Fish have historically lived in the Basilica Cistern, and some visitors have continued to report seeing them in recent years. However, the exact number of fish currently present has not been independently confirmed, and sightings are not guaranteed. The Basilica Cistern was originally constructed as a water reservoir, not an aquarium. Its underground chamber once stored enormous quantities of water for Constantinople's imperial district.

Today, the reservoir contains a much shallower layer of water beneath elevated visitor walkways. The reflections of the columns and changing illumination remain among its most recognizable features. Visitors who look carefully into the water may spot fish, although visibility depends on lighting, water conditions, and where the animals happen to be. The fish are an interesting detail of the Basilica Cistern's history, but the primary reason to visit remains its extraordinary Byzantine architecture.

Why Are There Fish in the Basilica Cistern?

The presence of fish in the Basilica Cistern is connected to its long history as an underground water reservoir. Built during the reign of Byzantine Emperor Justinian I in the sixth century, the cistern collected and stored water delivered through Constantinople's extensive water-supply system. Over time, fish became part of the underground environment. Their presence is documented in historical descriptions, but exactly when they first appeared and how they were introduced is not known.

Did Fish Enter Through the Ancient Water System?

One possibility is that fish entered through connections with the historical water network or were introduced after the reservoir was built. The Basilica Cistern received water from the larger system of aqueducts, channels, and reservoirs serving Constantinople. Such connections provide a possible explanation for how aquatic life could reach an underground reservoir.

However, there is no conclusive historical evidence identifying the exact route by which the first fish arrived. To understand how water reached this underground structure, explore the Byzantine Constantinople water supply system.

Were Fish Used to Test Water Quality?

A popular explanation suggests that fish were kept in historical water reservoirs as natural indicators of water quality. According to this theory, observing whether fish survived could provide a basic warning of problems in the water.

Although the idea is frequently repeated in discussions of ancient water systems, there is no strong historical evidence establishing that the Basilica Cistern's fish were originally introduced specifically for water testing. It is therefore better understood as an unconfirmed explanation rather than a proven historical fact.

The History of Fish in the Basilica Cistern

One of the most remarkable accounts of fish inside the Basilica Cistern comes from the sixteenth century.

Petrus Gyllius and the Fish Beneath Istanbul

In the 1540s, the French scholar Petrus Gyllius, also known as Pierre Gilles, travelled to Constantinople to investigate the city's ancient monuments.  During his research, he learned that residents living above an enormous underground structure could draw water through openings beneath their houses. Some residents even caught fish from the water below.

Following these accounts, Gyllius explored the underground reservoir by boat, moving between rows of ancient columns. His description of the Basilica Cistern includes fish living in its waters and local people catching them by torchlight. This account is important because it confirms that fish were present in the cistern centuries before the site became a modern tourist attraction.

Fish During the Ottoman Period

Following the Ottoman conquest of Constantinople in 1453, the Basilica Cistern continued to exist beneath the growing city. Although new water systems gradually reduced its importance, the underground reservoir was not entirely forgotten by local residents. People living above the cistern could still access its water through wells and openings. Historical accounts of fishing suggest that the reservoir supported aquatic life during at least part of this period. The fish therefore form part of the cistern's everyday history, connecting a monumental Byzantine structure with the lives of ordinary Istanbul residents.

What Kind of Fish Live in the Basilica Cistern?

Fish seen in the Basilica Cistern are commonly described as freshwater fish, particularly carp. Carp are associated with ponds, lakes, and slow-moving freshwater environments, which helps explain why they are often mentioned in descriptions of historical reservoirs.

However, the exact species currently present in the Basilica Cistern have not been established through a publicly available, up-to-date biological survey. For this reason, visitors should not assume that every fish visible in the water belongs to a particular species.

The most interesting aspect of these fish is not necessarily their species, but the fact that aquatic life has been associated with this underground structure for hundreds of years.

What Happened to the Fish During the Basilica Cistern Restoration?

The Basilica Cistern underwent a major restoration that was completed in 2022. The project focused on protecting the ancient structure while improving the visitor experience. Work included strengthening the building, conserving historic surfaces, replacing older concrete walkways, and introducing a new lighting system. These changes also transformed the appearance of the water beneath the columns.

Did the Restoration Remove the Fish?

There are differing accounts about what happened to the fish during the restoration process. Some accounts suggest that fish were temporarily relocated, while other descriptions and later visitor reports indicate that fish have continued to be visible in the restored cistern.

A complete, publicly verified record of the fish population before and after the restoration is not readily available. Therefore, it would be inaccurate to claim that all fish were permanently removed or that the historical population remained unchanged. What is certain is that the restoration changed the visitor environment, particularly the walkways, lighting, and appearance of the water.

Can Visitors See Fish in the Basilica Cistern?

Visitors may be able to spot fish beneath the walkways, but sightings should be considered a possibility rather than a guaranteed part of the experience. If you are hoping to see them, look carefully at the water between the columns while exploring the underground chamber.

Tips for Spotting Fish

• Observe the water during brighter phases of the changing lighting.
• Look beyond the strongest reflections, where movement may be easier to notice.
• Take your time along the designated walkways.
• Avoid using bright flash photography when observing the water.
• Never throw food or objects into the reservoir.

Remember that the Basilica Cistern is a protected historical monument, not a fish-feeding attraction.

Even if no fish are visible during your visit, the reflections, architecture, and ancient stonework make the underground experience memorable.

Why Is the Basilica Cistern Still Filled with Water?

The water inside the Basilica Cistern is an important reminder of its original purpose. Unlike an underground palace or ceremonial hall, the building was designed to store water for Constantinople. Its thick waterproof walls, vaulted ceiling, and carefully arranged columns allowed the structure to contain a large volume of water while supporting the buildings and streets above. Today, the remaining water contributes to the distinctive reflections that visitors associate with the Basilica Cistern.

The illuminated columns appear to extend below the surface, creating an almost endless underground landscape. This visual effect helps visitors understand why the monument is known in Turkish as Yerebatan Sarayı, meaning the Sunken Palace. Learn more about the underground structure in our Basilica Cistern architecture guide.

Discover the Basilica Cistern Beyond Its Fish

Although the underground fish have an unusual history, there is much more to discover inside the Basilica Cistern. The monument contains 336 columns arranged in 12 rows, with many stones reused from earlier Roman structures. Two famous Medusa head sculptures support columns near the far end of the chamber. Their unusual sideways and upside-down positions remain among the cistern's greatest mysteries. Discover the legends and possible explanations in our story of the Medusa Heads. Visitors can also discover the elaborately decorated Weeping Column and observe the combination of ancient architecture with modern lighting. To explore the monument and learn more about its history, visitors can book Basilica Cistern entry tickets with an audio guide or select a guided tour. Advance booking can help avoid the ticket-purchase queue, although all visitors must still pass through the required security controls.