Under Paris exists another world : the former quarries which were used to build Paris. They are more often called the catacombs, in reference to a very little part of this network where is stacked thousands and thousands of bones dating of the French Revolution.
There is approximately 180 km of galeries running 20meters under the streets of Paris which are the remaining parts of Paris quarries.
As every mysterious place, there is some legends.
The most famous is this story of the key keeper of the val de grace, Philibert Aspairt, who wanted to discover the treasure of Chartreux's monks. He decided to enter the former quarries on day of 1793. Nobody heard of him before 11 years.
I borrowed this note from (Guillaume Roche)
zoomed out version