Athens is usually experienced at street level or above: the Acropolis rising above the city, the hills framing it, the neoclassical façades and the lively scenes of urban life. Yet some of its most fascinating stories lie out of sight, beneath busy roads and squares, inside church crypts, forgotten basements and former wartime shelters. With civil engineer and speleologist Panagiotis Defteraios as our guide, we set out to explore an Athens that few visitors ever encounter.

In Athens, construction rarely proceeds without surprises. Every new metro station, building foundation or infrastructure project has the potential to uncover ancient walls, Roman aqueducts, buried waterways or remnants of long-vanished neighborhoods. In a city inhabited continuously for millennia, history does not exist only in monuments and museums; it often lies just a few meters below ground.

Most Athenians are aware of this layered past, even if they rarely stop to think about it. Every day, they walk over layers of history they may never see or be able to visit. Beneath the traffic of Vasilissis Sofias Avenue, for example, runs the concealed course of the Ilissos River. Once one of the city’s defining waterways, it flowed from the slopes of Mt Hymettus across Athens before joining the Kifissos. During the 20th century, large sections were covered by the modern street network, disappearing beneath roads such as Vasilissis Sofias and Vasileos Konstantinou avenues and Michalakopoulou, Ardittou and Kallirrois streets.

Water has shaped Athens for centuries in ways that are no longer immediately visible. At the foot of Lycabettus Hill in Dexameni Square lies the endpoint of Hadrian’s Aqueduct, one of the most important engineering works of Roman Athens. Built under Emperor Hadrian in the 2nd century AD, the underground system began on the northern edge of the city and ran for roughly 25 kilometers, supplying Athens with water for centuries. Today, like the Ilissos River, it forms part of an unseen geography that continues to exist beneath the modern metropolis.

But rivers and aqueducts are only part of the story. Hidden below central Athens are sections of ancient fortifications, cisterns, arcades, shelters and forgotten passages. Many remain inaccessible or difficult to identify from the street above. Perhaps that is precisely what makes them so compelling; they transform familiar places into something more mysterious, revealing layers of history concealed in plain sight.

“The unknown and the mysterious always attract us, especially when they involve things we cannot see,” says Panagiotis Defteraios, a civil engineer, speleologist, and author of Underground Athens (iWrite), one of the most comprehensive studies of the capital’s subterranean world. “It becomes even more fascinating when you’re walking down a street and discover the story hidden beneath it.”

In recent years, Defteraios founded Alter Exploring, an organization dedicated to introducing people to lesser-known corners of Athens through guided walks and excursions. Interest has grown steadily, and this summer the routes are being offered to international visitors eager to discover a different side of the city.

We asked him to recommend three sites in central Athens, all within easy reach of one another, that are safe and open to visitors. Together, they reveal a city that exists beyond the familiar landmarks, where Athens’ many histories persist below ground.

First Stop

The Themistoclean Wall beneath Dragatsaniou Street

At first glance, Dragatsaniou Street offers few clues that it conceals one of the oldest surviving boundaries of ancient Athens. Just a short walk from Klafthmonos and Korai squares, amid office buildings, cafés, bakeries and bars, a fragment of the city’s ancient fortifications lies hidden beneath an otherwise ordinary arcade.

The remains came to light during construction work in the 1970s, when excavations for the underground parking garage of Klafthmonos Square and nearby building foundations revealed sections of the ancient Themistoclean Wall. Rather than being removed or reburied, the masonry was preserved and incorporated into the modern structures above it, creating one of those uniquely Athenian encounters where antiquity and contemporary life exist side by side.

As you descend into the arcade’s basement, the sounds of the city gradually fade. Small shops, aging signs and shuttered storefronts hint at a commercial world that has largely moved on. Among them, ancient stonework emerges unexpectedly, pressed against concrete walls and structural supports. It’s not a monumental archaeological site, nor a carefully staged museum display. Its appeal lies precisely in its ordinariness: a quiet fragment of ancient Athens embedded below the modern city.

Built by order of the statesman Themistocles between 479 and 478 BC in the aftermath of the Persian Wars, these fortifications protected Athens for centuries and defined the limits of the ancient city. Roughly 6.5 kilometers long and boasting at least 13 gates, the wall combined stone foundations with mudbrick upper sections that, in places, reached eight meters in height.

The most substantial surviving section can be seen today in the archaeological site of Kerameikos. The experience on Dragatsaniou Street is very different. Here, the wall appears not within a formal archaeological landscape but in the basement of a contemporary building that people pass every day without realizing that traces of ancient Athens lie just below them.

Second Stop

The Cathedral Treasury beneath the Metropolitan Cathedral

Just steps from Syntagma Square and the winding streets of Plaka, another underground space offers a very different journey into the city’s past. Hidden beneath the Metropolitan Cathedral of the Annunciation – known simply as the “Mitropoli” to Athenians – the Cathedral Treasury houses a remarkable collection of ecclesiastical treasures rarely seen outside museum settings.

A staircase inside the cathedral leads to a purpose-built exhibition space below street level. Soft lighting, near silence and the faint sound of Byzantine chanting create an atmosphere of contemplation, regardless of a visitor’s familiarity with Orthodox Christianity. Within the display cases are embroidered epitaphioi, crosses, sacred vessels, vestments, votive offerings and liturgical objects, many distinguished by their exceptional craftsmanship.

Of particular interest are the texts in Karamanlidika, or Turkish written using Greek characters, which offer a glimpse into the complex cultural identities of Greek communities in Asia Minor. Richly illustrated holy texts, silverwork and devotional objects further illuminate centuries of religious and artistic exchange across the Orthodox world.

More than a repository of precious artifacts, the Cathedral Treasury tells a broader story about faith, migration, craftsmanship and the many networks that connected Greek communities across Europe and the eastern Mediterranean. The project was initiated by Father Thomas Synodinos, the cathedral’s presiding priest and a trained archaeologist, who sought to bring these long-hidden objects out of storage and into public view. With support from the Stavros Niarchos Foundation, the treasury opened to visitors in 2020.

Third Stop

Korai 4, an Underground Site of Memory

Just a few steps from Panepistimio metro station, at 4 Korai Square, a sign points toward the site of one of the most painful chapters in modern Greek history. Beneath the imposing Ethniki Insurance building, six meters below street level, lies the Korai 4 Historical Memory Site, a former wartime shelter transformed into a rare record of life under the Nazi occupation.

Designed by architects Anastasios Metaxas and Emmanouil Kriezis and completed in 1938, the building originally incorporated a state-of-the-art air-raid shelter. Following the German occupation of Athens in 1941, however, that underground space took on a darker role. Requisitioned by the German authorities, it was used by the Kommandantur as a detention center where prisoners were held before interrogation or transfer.

“It was primarily a place of temporary detention. Those considered more dangerous were transferred elsewhere. Some were taken to Kaisariani, where executions took place,” says Konstantinos Hatziyergatis, who has overseen the site since 2008. Following the liberation of Athens in 1944, the building briefly served as the headquarters of the National Liberation Front during the turbulent months that preceded the December ’44 clashes. Nearly half a century later, the former detention cells were transformed into a historical memory site, opening a rare window onto the years of occupation. Today, Korai 4 remains one of the few underground shelters in central Athens where visitors can still encounter much of the original infrastructure intact.

Passing through the iron doors, visitors encounter objects that belonged to the detainees: cigarette boxes, razors, cigarette cases and even notes that had been concealed in cracks in the walls. The display cases also contain the Nazi flag that once flew above the building.

In every room, large metal water tanks, toilets, ventilation systems and iron doors are still in place. Some of the doors, manufactured in Germany between 1936 and 1939, retain traces of their original markings. “In some places, you can still make out the word ‘Berlin,’” notes Panagiotis Defteraios. “The word appears to have been deliberately defaced by detainees.” The most powerful part of the visit lies deeper underground, in the former detention cells. Here, the walls preserve hundreds of inscriptions scratched by prisoners using whatever tools they could find. Names, dates, messages and fragments of testimony survive as direct voices from the Occupation: “24 hours without food or water. Only smelling the jasmine. Dimitris Moraitis.” “Mavrikios N. Malevris, final-year medical student. Held as a hostage. Tortures of the Holy Inquisition.” “The Halaros brothers. Honest citizens. Detained unjustly after being denounced by an informer.”

Most inscriptions date from 1944, the final months of the occupation. According to Defteraios, such messages were often covered with whitewash so they’d stay unseen by subsequent prisoners. Among those surviving are sketches, caricatures and symbolic drawings, including one thought to mock Mussolini and still bearing bullet marks.

The Korai 4 site isn’t simply a museum. It’s a place that demands time, attention and silence. Descending into its underground rooms means entering an Athens marked by occupation, fear and resistance. Emerging once again into the light of Korai Square, the distance between past and present suddenly feels much smaller.