(Top left) Paris Metro escalators. (Top right) Paris Metro stairs. (Bottom) Subway conveyor elevator.

(Top) A view of the Paris sewers. (Bottom) A view of the Paris sewers. The underground is a pilgrimage site for visitors to Paris. Since the metro is the city's primary transport, visiting Paris without riding it feels incomplete. Thus, experiencing Paris begins with the subway. For centuries, Paris was a major producer of stone and limestone, leading to extensive quarrying. Today, numerous underground limestone corridors remain beneath the modern metropolis, alongside the metro, water reservoirs, drainage systems, gas lines, and telecommunications cables. The ground seems hollowed out, which is hard to imagine. The underground is divided into three layers: first the metro, then the water and drainage systems, and finally the catacombs, extending up to 20 meters deep.