Definitely not for the faint of heart, the city’s catacombs are filled with the remains of millions of ex-Parisians, whose bones line the narrow passages of this mazelike series of tunnels. In the 18th century, the Cimetière des Innocents, a centuries-old, overpacked cemetery near Les Halles, had become so foul and disease-ridden that it was finally declared a health hazard and closed. The bones of its occupants were transferred to this former quarry, which were later joined by those of other similarly pestilential Parisian cemeteries.

In 1814, the quarry stopped accepting new lodgers. Rather than leaving just a hodgpodge of random bones, they organized them in neat stacks and geometric designs, punctuating the 2km (1 1/4 miles) with sculptures and pithy sayings carved into the rock. The one at the entrance sets the tone: stop—here is the empire of death. The visit will be fascinating for some, terrifying for others; definitely not a good idea for claustrophobics or small children. You’ll want to wear comfortable shoes and bring a sweater of some sort, as it’s cool down here (around 57F/14C).