Every day, more than five million passengers use the Paris metro, more than twice the population of the capital. This transportation network, which resembles an anthill, never sleeps. In the morning, the drivers of the first train take over from the maintenance workers who work in the quiet of the night. The manufacture of the metro is the lifeblood of nearly a dozen industrial sites in France. Opened to the public in 1900, this underground world is a major piece of French heritage. Its history will accelerate with the creation of the Grand Paris Express project, which will double the existing network.