Beneath the streets of Athens lies a largely unseen world of buried rivers, ancient aqueducts, underground tunnels, air-raid shelters and sprawling sewer networks. Built up over centuries, these structures coexist with the modern city, yet much of the underground landscape remains fragmented across archives, technical studies and incomplete records rather than mapped in a single, comprehensive system.
The lack of a complete picture is more than a historical or archaeological concern. As Athens expands its Metro network, excavations through complex geological formations and areas containing potentially undocumented structures can create unexpected engineering challenges. Thorough geological and geotechnical studies are essential to identify potential hazards before construction begins and reduce risks that could affect both infrastructure and public safety.
The underground challenges beneath Kypseli
The Athens neighborhood of Kypseli illustrates the difficulties involved in building beneath the capital. Beneath its densely populated streets lie complex geological formations, traces of old streams originating in the Tourkovounia hills, and structures that have been enclosed or covered during urban development.
Serious ground subsidence recorded during Metro construction since March 2026 has highlighted these risks. The incidents have raised concerns about the structural integrity of several apartment buildings, underscoring the need for more accurate underground mapping and detailed geotechnical investigations before major infrastructure projects proceed.
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Efthymios Lekkas, professor emeritus of Dynamic, Tectonic and Applied Geology and Natural Disaster Management at the National and Kapodistrian University of Athens, and president of Greece’s Earthquake Planning and Protection Organization (OASP), told To BHMA that a more complete understanding of underground conditions improves the ability to anticipate and manage risks.
However, he warned that geological studies are often treated as an expensive stage of construction that can be scaled back to accelerate projects.
Designing Metro lines in Athens presents a particularly complicated geotechnical challenge because the routes pass through areas with different geological characteristics.
“The Metro has to cross an extremely difficult area,” Lekkas said, explaining that excavations encounter Alpine rock formations, post-Alpine formations and deposits created by human activity.
A city shaped by 2,500 years of construction
Natural geology is only part of the challenge. The underground environment may also contain old wells, foundations and other structures that have not been adequately documented.
Lekkas cited the collapse of a kiosk on Panepistimiou Street in 1997, when a tunnel-boring machine passed beneath the area. According to him, an old well had not been identified at the site.
Human activity has left a substantial imprint on Athens’ underground landscape. With approximately 2,500 years of continuous habitation, the city has undergone successive phases of construction, demolition and redevelopment. Artificial fill, abandoned structures and other human-made deposits now lie alongside natural geological formations, adding complexity to underground investigations and construction.
Together, these conditions make comprehensive geological and geotechnical studies, along with more complete records of underground structures, essential for the safe execution of major infrastructure works.
Building a centralized geological database
Despite the challenges, Attica’s underground environment is the most extensively studied in Greece, according to Dionysios Goutis, General Director of the Hellenic Survey of Geology and Mineral Exploration (H.S.G.M.E.).
The difficulty, he said, is that the large volume of geological and geotechnical information collected over the years has not yet been consolidated into a single, comprehensive recording system.
H.S.G.M.E. has developed GeoPortal, a geological data platform that incorporates its Geotechnical Data Geodatabase. The initiative is intended to help collect, organize and make better use of the information available about underground conditions.
Its effectiveness, however, depends on continuous updates and cooperation with public and private organizations that hold valuable geological and geotechnical data.
The goal is to gradually develop a more complete and reliable picture of the ground beneath the capital. This would allow major engineering and infrastructure projects to be planned and carried out on the basis of stronger scientific evidence, reducing technical uncertainty, risks and associated costs.
A hidden city of shelters, tunnels and aqueducts
Athens’ underground world also preserves traces of its history, including air-raid shelters built in the 1930s, tunnels and passageways from different periods, caves, ancient aqueducts and rivers that have been routed underground. The Ilissos River is one example. The underground sewer infrastructure serving the Athens basin is another major part of this hidden network.
Konstantinos Kyrimis, an author and researcher specializing in shelters and passive air defense during World War II, has been documenting many of these structures.
“There are many structures beneath our feet,” he said, pointing to a large shelter covering 250 square meters just 50 meters from Syntagma Square, in the direction of Monastiraki.
Although largely invisible in everyday urban life, such structures form part of Athens’ historical development and illustrate how much remains concealed beneath its streets.
Traces of an ancient water network
Another example is the reservoir in Kolonaki, where remnants of an old aqueduct can still be found.
At the far end of the site, an entrance leads to an underground tunnel connected to the aqueduct. The accessible section extends for about 50 meters before being blocked by sediment and other debris.
Kyrimis said the tunnel once continued farther and branched into a wider network that originated on Mount Penteli and Mount Parnitha.
The remains offer a glimpse of an underground water system that once extended well beyond the section that can be accessed today.
Athens’ wartime air-raid shelters
Among the city’s notable underground structures are the air-raid shelters on Ardittos Hill and Mount Lycabettus.
The shelter on Ardittos was built shortly before Greece entered the war in 1940 and is the largest in Athens, with a capacity of more than 1,300 people.
The Lycabettus shelter was created in 1936 and extends approximately 100 meters into the rock. During the war, it housed the capital’s Air Defense and Alert Center.
“It was the city’s ‘eyes’ and ‘ears,’” Kyrimis said, describing the role the facility played in monitoring threats and coordinating alerts.
These shelters, like the city’s aqueducts, buried waterways and forgotten structures, form part of an extensive underground landscape that remains largely out of sight even as modern Athens continues to build above it.