The catacombs in Rome were underground tunnels and burial galleries for Christian martyrs in ancient Rome. Christians would recognize the contributions of these martyrs by attending ceremonies in the catacombs on the anniversaries of the deaths of the martyrs.
The Catacombs in Paris, France are a series of tunnels converted into a mass grave during the 18th century in order to alleviate the problem of overcrowding in the cemeteries in the city. The monks responsible for cleaning and arranging the remains organized them into piles and patterns ranging from Xs to crosses. There are estimated to be the remains of 5 to 6 million people in the Parisian catacombs. The tunnels are a popular tourist spot today, however there are sometimes issues with tourists stealing a bone or two as a souvenir.