Even France's favorite aquatic son Jacques Cousteau, couldn't get to the bottom of it.

An unusual sand beach in the middle of a forest.

Abandoned railway line circling the city of Paris.

A cave dwelling that was started by the Celts, expanded by monks, and finished by knights.

Home to fantastic views and ancient graves.

This fascinating abbey is considered the oldest monastery in the region.

The castle where the Marquis de Sade lived out his darkest fantasies.

Series of underground houses inhabited continuously for centuries.

These medieval ruins have survived since the 12th century, even providing shelter from the Nazis.

Nicknamed the "Museum of Horrors," this French anatomical collection has been fascinating visitors for over a century.