Archaeologists have removed roughly 3,000 tons of debris from a centuries-old network of underground tunnels beneath Istanbul's iconic Hagia Sophia — revealing previously inaccessible passages beneath one of the world's most historic religious landmarks.
The findings were announced by archaeologist Hasan Fırat Diker at a presentation this fall in Istanbul, as local outlet Türkiye Today reported.
The tunnels have been undergoing excavation and clearing work since February 2025, allowing researchers to explore sections that were previously buried beneath mud and debris.
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During the Oct. 16 presentation, Diker said the roughly 1,600-year-old tunnels were built by the Romans and originally functioned as part of the city's water distribution and drainage network, while also providing a protected underground space.
Some of the underground structures predate the present-day Hagia Sophia, which was completed in 537 A.D. under Byzantine Emperor Justinian I.
Although the underground network has been known for decades, much of it has remained difficult to access, and the ongoing excavation work is allowing researchers to examine previously buried sections of the historic site.
Diker's research has shed new light on the intricate network, which he compared to a modern subway system, Türkiye Today said.
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The tunnels were designed to carry rainwater from surrounding streets into the sea, according to Diker.
Researchers have cleared nearly 558 feet of an underground route connected to a fifth-century Roman street, which extends from the Sarayburnu area toward Little Hagia Sophia, Diker said.
The landmark, part of a UNESCO World Heritage Site and visited by millions of tourists each year, is actually the third structure built at the site, Diker said.
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He said his team had cleared both the upper and lower sections of an ancient underground burial complex discovered in 1946, excavating an additional five feet beneath it.
The burial chambers predate the current Hagia Sophia; they had accumulated large amounts of mud over the centuries, according to Diker.
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Looking ahead, Diker said researchers hope to open more sections of Hagia Sophia's underground network to visitors, with plans for an exhibition showcasing the archaeological remains.
Hagia Sophia — which means "Holy Wisdom" in Greek — served as a major center of Eastern Christianity for nearly 1,000 years until Ottoman forces conquered then-Constantinople and converted the church into a mosque in 1453.
It later became a museum in 1935 before being converted back into a mosque in 2020.
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The excavation is the latest in a series of archaeological investigations involving ancient underground tunnels around the world.
Earlier this year, archaeologists uncovered a mysterious 164-foot-long tunnel in Jerusalem near a site linked to the biblical Kingdom of Judah, though its age and purpose remain unknown.
In Germany, archaeologists discovered a medieval tunnel beneath a prehistoric burial site after initially mistaking its entrance for a grave.