So, you know how we bury people when they die, right? Well a really long time ago in some places, like in Paris, France, they had so many people dying all the time that they didn't have enough room to bury everyone in the ground.
So they started building tunnels and rooms underground to store all the bodies. These underground tunnels and rooms are called catacombs.
People used to go visit these catacombs and see all the bones and skulls that were stacked up in there. It was kind of like a spooky museum! Nowadays, people mostly go visit the catacombs to learn about history and see how people used to handle lots of deaths. But it's still pretty cool to go down there and see all the bones!