The history of the Paris catacombs, the city's underground tunnels of death home to over 6 million corpses

• The Paris catacombs were constructed after mass gravesites started to pollute the soil and leech smells. 
• The catacombs were originally the site of limestone mines used during the Roman Empire. 
• It took almost 30 years to move 6 million corpses into the catacombs. 

Mass graves had been in use for centuries in Paris, reaching a point where some graves held 1,500 bodies at a time. The smells and gasses were so bad that, in one instance, three gravediggers died from the gasses in a church vault.