Meet your guide near the Catacombs entrance at Denfert-Rochereau and get ready to head far below the city. Descend the long spiral staircase into a maze of tunnels first carved as limestone quarries in the Middle Ages, later used as burial chambers when the city's graveyards overflowed. As you walk the bone-lined corridors, you'll see skulls and femurs stacked floor to ceiling and arranged into crosses, hearts, and archways, while your guide explains how the remains of millions came to rest here. Next, stand before the Barrel of Passion in the Crypt of Passion, the most striking arrangement underground, and hear the tale of a secret midnight concert once held in these tunnels. The highlight comes when a guard unlocks the iron gates to two restricted chambers closed to the general public, where your small group sees rooms most visitors never reach. The tour ends deep inside the Catacombs, with free time to linger and explore at your own pace before you exit to street level.