Europe is home to a number of mega-cities , whether it’s London,Berlin or Athens, each have insanely big populations and they seem to be growing every year. But there is one capital on the old continent that is not only one of the biggest, but also officially the most popular.

That would be Paris, France, which boasts the title of the most visited city in Europe. According to figures from AXA, it welcomes more than 30 million tourists a year, more than any other city in Europe. People flock there to see famous landmarks like the Eiffel Tower, the Louvre, and Notre Dame Cathedral.

Paris is also enormous. The current metro population in 2025 stands at 11.3 million, according to Macro Trends. 

But despite all the beauty above ground, there’s something terrifying underneath the city.

Deep below the streets lies a chilling maze known as the Catacombs. It’s a network of tunnels holding the remains of around six to seven million people. 

While many tourists often miss it, it’s one of the most haunting parts of the French capital.

The underground passages were originally limestone quarries, but by the late 1700s, Paris faced a serious crisis.

Cemeteries were overflowing and the smell of decomposing bodies was affecting homes and businesses nearby.

According to the official Catacombs website, the largest cemetery, Saints-Innocents, had been used for nearly 1,000 years. 

So authorities made a bold decision to move the bones to the abandoned Tombe-Issoire quarries.

Starting in 1785, workers moved remains into the tunnels at night to avoid upsetting the public. 

The process took decades, especially after the French Revolution and later during the city’s redevelopment under Napoleon III in the 1850s.

Nonetheless, the tunnels were officially consecrated in 1786 and named the Paris Municipal Ossuary. 

But they soon became known as the Catacombs, in reference to similar burial chambers in Rome.

The full tunnel system stretches under much of Paris (around 800 hectares) but only a small section is open to the public. 

Inside, skulls and bones are stacked neatly along the walls, with plaques bearing messages about life, death, and the fragility of existence.

Tours began by appointment in the early 1800s, and the site officially opened to the public in 1809. 

Today, around 550,000 people visit the Catacombs every year, entering through 1 Avenue du Colonel Henri Rol-Tanguy.

The tunnels are strictly monitored, and only designated routes are accessible. 

Parts of the network have collapsed over time, and maintaining the site is an ongoing challenge due to humidity and decay.