The Hagia Sophia Grand Mosque has attracted thousands of visitors since it was reopened as a place of worship after being reverted from a museum in 2020. But few people are aware that beneath the architectural wonder at the heart of Istanbul lies a network of tunnels, cellars and graves.

Professor Hasan Fırat Diker and his team, meanwhile, continue exploring "underground Hagia Sophia," examining the intricate structures beneath the building that was reconstructed three times in its history dating back to Byzantine times. Diker is an academic from the Faculty of Architecture and Design at Fatih Sultan Mehmet Vakıf University (FSMVÜ), which was established by a foundation originally started by Mehmed II, the Ottoman sultan who conquered Istanbul and converted the Hagia Sophia into a mosque from a cathedral.

Two years ago, researchers mapped the tunnels credited with keeping Hagia Sophia from falling victim to excess moisture by serving as a natural ventilation system. They created three-dimensional imagery of the tunnels whose length reaches about 1 kilometer.

Along with the tunnels, a three-chamber tomb resting 4 meters beneath the ground is located in the northeastern section of the mosque. It dates back to the fourth century A.D. according to historians.

Some tunnels and cellars are accessible from a trapdoor under a chestnut tree southwest of Hagia Sophia, just 2 meters below the surface.

"Just like above, the Hagia Sophia has a rich history under the surface," Diker says. "Four meters below the surface, there is a ‘hypogeum’ (tomb) and it was built long before the construction of Hagia Sophia. It is actually the oldest architectural structure found in the immediate vicinity of Hagia Sophia and was linked to other underground structures by adding culverts in later years," he said.

"We aimed to document the infrastructure of Hagia Sophia, as well as its superstructure, in an architectural sense and to raise awareness for the elimination of existing problems," he told Anadolu Agency (AA) on Monday.

Researching, preserving and documenting the underground structures is important both in terms of protecting the building and revealing the other underground structures of the city it is related to, he noted.

"The construction of Hagia Sophia, which has survived until today, started in 532 and was completed in 537," he said, adding that the main space in this building is covered with bricks on four arches supported by four piers.

The Hagia Sophia was exposed to uncontrolled rainwater before it was cleaned, he said, adding that some four tons of rubble and mud were removed within the scope of the cleaning work carried out last year.

Noting that there are cellar structures 2 meters below the garden in front of the northwest facade, he said these are the infrastructure spaces from the original atrium of the Hagia Sophia.

Most of the underground spaces of Hagia Sophia were built for plumbing and ventilation, he said, adding that it is difficult to cross the tunnels due to how narrow they are.

"Even breathing is not easy in these tunnels ... When you think about the time they were built, we can understand how difficult it was for people to move in these dark places and it is not possible to use them for any other purpose," he added. He says the fact that the tunnels are so difficult to squeeze through shoots down the "urban legend" that the tunnels below the Hagia Sophia extended as far as the Maiden’s Tower and even the Princes’ Islands kilometers away in the Marmara Sea.

Diker believes that the grouping of structures beneath the Hagia Sophia were likely used as storage depots for "materials belonging to the place people did not want to be exposed." "These places need serious restoration," he added.

Hagia Sophia served as a church for 916 years and as a museum for 86 years, but from 1453 to 1934, nearly 500 years, it was a mosque. In 1985, the Hagia Sophia was added to the UNESCO World Heritage List. It is among Türkiye's top tourism destinations and remains open for domestic and foreign visitors.