The Left or south bank of the Seine in Paris contains large deposits of limestone. From the 12th century onwards, these deposits were mined to produce building materials for the expanding city of Paris. Mining activity was haphazard and mostly undocumented.
As the city expanded outwards, buildings were erected over the former mines. From the 17th century, the network of mines below the city began to cause problems for the buildings above. A Commission of Mines was appointed to investigate and shore up the myriad tunnels beneath the city.
At around the same time, the cemeteries within the city boundaries were under considerable pressure and it was decided to use a part of the old mine workings to store the contents of the city cemeteries. The whole process, taking place mostly under cover of darkness took approximately two years to empty the Paris cemeteries.
The catacombs contain the bones of over 6 million people.
At the beginning of the 19th century, the bone repository was converted to a visitable mausoleum or ossuary. A visit involves a small number of stairs and nearly 2 kms of walking.
The Catacombs are open daily except for Mondays and some holidays.
…and you will get your bags searched on the way out!
Details: Catacombs of Paris