In 2004, while workers were constructing a new tram line between Pont Michel and Las Planas in Nice, they stumbled across something odd: fragments of an old medieval city, hiding in a 6,000-square-foot chamber just beneath the ground. 

The crypt’s entrance is right next to the tramway that runs parallel to Place Jacques Toja in Old Nice, where a small trapdoor opens to reveal a staircase. An elevator bearing a poster advertising “La Crypte de Nice” is the only clue revealing what lies just 20 feet below the ground. During off-hours even the staircase disappears. A panel seals the entrance, making it blend in with the rest of the square.

The secret subterranean spot has been open to the public since October 2012. Sightseers are first greeted by a map of the old city, complete with a red circle that indicates the location’s past life as an old wall that once formed the barrier between Nice and the surrounding land. These old walls and a well-preserved 14th-century tower are among the first visible artifacts. Bits of medieval houses that belonged to the Augustin family and an aqueduct that once brought water to the Sardinian King’s palace lie deeper within the crypt. This area also holds the remnants of a bridge that led to Turin, Italy and a 17th-century moat built to protect the then-independent Nice from French invasions.

Plaques with illustrations are scattered throughout the underground city to help visitors imagine how the place might have originally looked. But even without their help, it’s still pretty impressive to see history lying just below the unsuspecting pedestrians perusing the streets above.