Beneath the streets of Paris lies a forbidden underground world: nearly 300 kilometers of tunnels that run deeper than the city's sewers and metro lines. These mysterious passageways bear hidden traces of key moments in history — from the French Revolution and the Paris Commune to the failed coup of 1937 by the Cagoule, the German occupation, and Cold War nuclear shelters. The catacombs stretch 15 to 25 meters below ground, sometimes across two or three levels. Originally ancient stone quarries, they provided the materials to build many of the city’s iconic monuments. Today, these underground tunnels offer a rare glimpse into a forgotten Paris — a city whose hidden stories remain buried beneath the surface. Officially, only a small portion of the catacombs is open to the public: about 6 kilometers beneath the Left Bank, housing the remains of six million Parisians from the city's former cemeteries. But beyond this section, hundreds of kilometers of tunnels remain off-limits. Adventurous explorers, known as cataphiles, defy the authorities to venture inside, often accessing the network through manhole covers and hidden shafts that are frequently sealed off or reinforced. Entry to these dark passages has long been forbidden. Yet throughout history, people have used them to hide, escape, or move discreetly through the city during turbulent times. The catacombs played a role during the French Revolution, the Paris Commune, and other uprisings — even earning a mention in 19th-century literature and more recently in Umberto Eco’s The Prague Cemetery. In more modern times, these tunnels served a strategic purpose. In 1937, the far-right paramilitary group known as the Cagoule attempted a little-known fascist coup from the underground. During World War II, the German occupiers built air-raid bunkers in the tunnels, while members of the French Resistance used them as a hidden headquarters during the city’s liberation. In the Cold War era, secret nuclear shelters were quietly built beneath public buildings, far from the public’s knowledge. Since the 1980s, the catacombs have become the setting for clandestine parties, where small groups of students and thrill-seekers evade authorities and navigate this forbidden underworld — even as the entrances are periodically sealed shut. Exploring the Paris catacombs means diving into centuries of forgotten stories and hidden histories — an underground city that holds the secrets of a tumultuous past.