In Paris, you can visit the catacombs at Place Denfert-Rochereau. This is one of the most extraordinary places to see in Paris, although it is not for the faint of heart.
At the end of the 18th century and during the 19th century, millions of skeletons were taken out of the cemeteries in Paris because the neighbors complained about the smell, and they were stored here. The result is macabre.
Imagine that you are walking between two walls made of bones. The bones are very well ordered: big bones are separated from little bones, and skulls form regular patterns in the middle of the wall. It would look great in your living room. All of these skulls and bones belong to dead people. And behind them, you know that there are other bones and skulls, ten or twenty meters full of broken and disordered human skeletons.
At the end of the walk, you get out of the underground in a little unknown street very far away from the entrance, and you are completely lost. You have seen more dead people in 20 minutes than a medical examiner during his whole career.