Most people have heard about the catacombs of Paris but not many realize that the catacombs make up a tiny portion of a massive quarry complex that exists under the city. The urban explorers that go into these quarries are called cataphiles. The video below was made by one of these cataphiles.
The quarries became a problem as Paris expanded. In 1774 there was a major street collapse that caused the king to organize a special group whose job was to map the underground quarries and shore up the foundations of buildings built over them. You can still see the markings they made when erecting supporting columns under the city.
This is one of a series of videos made by this explorer. Be sure to watch them all.