Some sociologists assert that the sophistication of a society can be judged by the way it disposes of waste. If so, Paris receives good marks for its mostly invisible sewer network. Victor Hugo is credited with making them famous in Les Misérables: Jean Valjean takes flight through them, "all dripping with slime, his soul filled with a strange light." Hugo also wrote, "Paris has beneath it another Paris, a Paris of sewers, which has its own streets, squares, lanes, arteries, and circulation."

In the early Middle Ages, drinking water was taken directly from the Seine and wastewater poured onto fields or thrown onto the then-unpaved streets, transforming the urban landscape into a sea of rather smelly mud. Around 1200, the streets were paved with cobblestones, and open sewers ran down the center of each. These open sewers helped spread the Black Death, which devastated the city. In 1370, a vaulted sewer was built on rue Montmartre, draining effluents into a Seine tributary. During Louis XIV's reign, improvements were made, but the state of waste disposal in Paris remained deplorable.

During Napoleon's reign, 31km (19 miles) of sewer were constructed beneath Paris. By 1850, as the Industrial Revolution made the manufacture of iron pipe and steam-digging equipment more practical, Baron Haussmann developed a system that used separate channels for drinking water and sewage. By 1878, it was 580km (360 miles) long. Beginning in 1894, the network was enlarged, and laws required that discharge of all waste and storm-water runoff be funneled into the sewers. Between 1914 and 1977, an additional 966km (600 miles) were added. Today, the network of sewers is 2,093km (1,300 miles) long. It contains freshwater mains, compressed air pipes, telephone cables, and pneumatic tubes. Every day, 1.2 million cubic meters of wastewater are collected and processed by a plant in the suburb of Achères. One of the largest in Europe, it's capable of treating more than 2 million cubic meters of sewage per day.

The city's égouts are constructed around four principal tunnels, one 5.5m (18 ft.) wide and 4.5m (15 ft.) high. It's like an underground city, with the street names clearly labeled. Each branch pipe bears the number of the building to which it's connected. These underground passages are truly mammoth. Sewer tours begin at pont de l'Alma on the Left Bank, where a stairway leads into the city's bowels. However, you often have to wait in line as much as half an hour. Visiting times might change during bad weather, as a storm can make the sewers dangerous. The tour consists of a film, a small museum visit, and then a short trip through the maze. Be warned: The smell is pretty bad, especially in summer.