Where are the "ghost stations" that are never open to the public? What really wacky subway projects have Parisians escaped from? In which corridor do we marvel at the superb earthenware of the North-South? Why have some stations lost their baptismal names? What do the mysterious signals of the trains and tunnels mean? The Parisian metro conceals many unsuspected treasures that tell us as much about the history of railway technology as it does about the history of the capital. But for lovers of unusual discoveries, the melody of the underground is above all an invitation to travel. All aboard!