The official catacombs in Paris are open to visitors all year long. It’s a very small section of what the actual network of underground corridors is. In this section were grouped all the remainders (bones, skulls) from the cemeteries that were located in the medieval Paris and that have been removed over the past centuries. The demographic pressure made it necessary to free more space inside Paris to allow the city to grow.
Now once you get down there, be warned, it’s a particular atmosphere. You wander along perfect arrays of millions of bones, all nicely lined up and forming litterally walls. Looks pretty decorative but indeed, it’s kinda special. It’s not a small walk but well, you won’t see time flying by.