cerephinna

The Catacombs were created in the late 18th century when the cemetaries of Paris became so crammed that those living nearby became diseased and the ground level had risen 10-20 feet from the volume of human remains buried there.
From 1786 til 1814 the bones of about 6 million exhumed bodies were put into the ancient quarries of Paris, linked by several passages ranging in size and depth and totalling almost 300km.
By the time you get to the bottom of the first flight of stairs you're already 25m under the Paris streets.
The walls themselves are lined with 2-3 metres of human bones, typically femurs with a row of jawless and toothless skulls on top.
The Catacombs were used both by the Germans and the French Resistance during WW2.