There are over 300 kilometres of underground passages underneath Paris, France as underground quarries were made due to the large demand of stone it took to build the city. The catacombs beneath Paris hold nearly 6,000,000 (6 million) human skeletons. During the 18th century, death was quite common and the cemeteries quickly filled up. As such, the city of Paris decided to empty the cemeteries and place the bones underground. The bones were tidied up by making them into walls by stacking skulls and humerus arm bones on top of eachother, making each piece fit in the groove of the next. The collection of bones beneath Paris is essentially the Medieval population of Paris