Paris has hundreds of kilometres of hidden tunnels beneath its streets.
What are they mainly?

Question

Accepted Answer

Abandoned limestone quarries — also housing the Catacombs, with the remains of 6 million people. — The limestone made Notre Dame; the empty tunnels became the world’s largest ossuary. Below Paris is a mirror-city of bones.

Paris has hundreds of kilometres of hidden tunnels beneath its streets. What are they mainly?