These are the Paris Catacombs: a network of old caves, quarries and tunnels stretching hundreds of miles, (In total the winding Catacombs stretch over 300 kilometers (186 miles) and seemingly lined with the bones of the dead.
Layers and layers of skeletons were piled high and wide along the walls. The Catacombs of Paris is the final resting place of millions of bones.
Due to overpopulation and lack of sanitary conditions around cemeteries, the water became contaminated and became a source of infections around the area.
In the 18th century the bones of Paris' dead were moved from their cemeteries to this abandoned stone mine.
Although the Paris Catacombs are still open to the general public today, access is limited to only a small fraction of the network. It has been illegal since 1955 to enter the other parts of the catacombs. More information at ancient-origins.