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.
Original inscriptions from when the tunnel was used in the eighteenth century are etched along the walls.
The catacombs have been a morbidly fascinating tourist attraction in the city for years.
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.
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.
Several signs that date back tell of the different cemeteries the bones come from.
The Catacombs are open for tours Tuesday to Sunday. Tours are about 45 minutes long and cost 10 euros for adults.