Catacombs of Paris

Wikipedia - "The Catacombs of Paris or Catacombes de Paris is a underground ossuary in Paris, France. Located south of the former city gate (the 'Barrière d'Enfer' at today's Place Denfert-Rochereau), the ossuary holds the remains of about six million people[1] and fills a renovated section of caverns and tunnels that are the remains of Paris's stone mines. Opened in the late 18th century, the underground cemetery became a tourist attraction on a small scale from the early 19th century, and has been open to the public on a regular basis from 1874. Following an incident of vandalism, they were closed to the public in September 2009 and reopened 19 December of the same year."
Wikipedia
National Geographic: Paris Catacombs
National Geographic: Under Paris
National Geographic: maps
CNN - Bone people: The explorers of the Parisian catacombs (Video)
“Scariest Places on Earth”: The Paris Catacombs (Video)
YouTube: The Empire of Death - Paris Catacombs, Inside the Catacombs of Paris, Paris Catacombs Exploration 2009