Musée des égouts de Paris
Paris (not the one in Texas!), the most romantic city in the world, is also home to the world's first and longest underground sewer system.
It all started in the middle age, when King Phillipe Auguste incorporated a drain for waste water in the middle of the paved roads. But it was still above ground. Underground tunnels weren't introduced until the reign of the great Napoleon Bonaparte.
Today, the sewer network is more than 2,100 km long. Before 1970, tourists used to be able to go down the sewer by carts. They switched to boat tour after 1970. Today, you don't need any of those.
The Paris Sewers Museum, located in the sewers for your convenience, is open to all visitors. It's full of interesting displays, with descriptions in English explaining the evolution of the tunnel networks. A must see for those who are into engineering. Oxygen mask is highly recommended.
