Hidden beneath the vibrant European city of Paris is an eerie network of old caves and tunnels housing the remains of 6 million people: the Catacombes de Paris.
The tunnels, located beneath the Palais de Chaillot, stretch as far as about 200 miles, though only a small section of it is open to the public today.
The majority of the network has been off-limits since 1955, but even in the area still accessible to tourists are fascinating remnants of Paris' history.
The limestone quarries were used as a natural resource since the time of the Romans, providing construction material for Paris' buildings. Eventually, the city continued to expand over the quarries, creating a vast underground world.
In the 18th century, when Parisian cemeteries like its largest, Cimetière des Saints-Innocents, became overpopulated, six million bodies were both transferred from cemeteries and buried directly into the underground limestone passageways, giving the catacombs a reputation of the world's largest grave.
Since they were officially opened to the public in 1876, the catacombs have attracted the interest of thousands of visitors and historical figures like King Charles X, who is said to have thrown wild parties there, and Napoleon III.
They were used by the French Resistance to hide from Nazis in World War II, and ironically, by the Nazis to built an underground bunker.
In some places, bones are laid out in careful patterns, stacked high on top of one another.
Other areas include a sea of fragmented skeletons along the floor.
There is also graffiti dating all the way back to the French Revolution etched along portions of the wall.
Famous figures buried here include Jean-Paul Marat, known from the French Revolution, and Maximilien de Robespierre, an iconic member of both the Revolution and the Reign of Terror.
Unofficial visits are not permitted considering how dangerous this can be, but there are secret entrances across the city , like sewers and metro tunnels, that lead into the site.
There is even an entire group of explorers known for entering the network illegally. The Cataphiles have been going deep underground the catacombs for years, hosting everything from bars to movie screenings in the subterranean world.
Today, about a mile of the Catacombs is open to explore, with a public entrance in Paris' 14th arrondissement.
There is also an area that houses a themed exhibition each year, with this year's showcasing the history of Paris when it was occupied by the Lutetian sea about 45 million years ago.
The Catacombs are open for tours Tuesday to Sunday. Tours are about 45 minutes long and cost 10 euros for adults.
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