Clearly on the creepy side as far as Parisian attractions are concerned, the Catacombs are old quarries that contain about six million skeletons moved here between the end of the 18th and the middle of the 19th centuries from overcrowded cemeteries.
Despite their morbid character (or, perhaps, because of it), the Catacombs are very popular - they welcomed over 253,000 visitors in 2007 (source: Mairie de Paris). The Catacombs underwent 3-month long renovations in 2008 and reopened at the end of April, 2008.
Only a small portion of the Catacombs is open to visitors, although there exists a subculture of those who enjoy visiting (illegally) the non-public portion. We do not advocate joining that club.