The curious underground history of keeping Paris clean.

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

In the best of all possible worlds, everyone will get a chance to see this statue in which Voltaire's heart is interred.

Exquisite wall sculptures carved in secret by an early quarryman in an obscure corner of the Paris Catacombs.

A majestic lion statue guards this Parisian botanical garden, and feeds on an unsettling item.

The ruins of a Duke's bucolic dream, where camels once roamed alongside "Dutch" windmills and faux-Italian vineyards.

Indents in the pavement are all that remain from a grisly death machine in Paris.

A haunting collection of the vanished and disappearing natural world.

The very center of Paris is surrounded by a bevy of strange good luck rituals.

These temples are among the oldest free-standing stone buildings in the world.

Two geological phenomena in one precarious locale: a towering stone window, and a deep blue void.

Hidden beneath a restaurant, underground tunnels tell the story of Malta’s tragic role in World War II.

The fishing boats that moor in this small lagoon enter and leave through a tunnel to the sea.

This almost-forgotten underground dungeon housed several convicts over the years, including Caravaggio.

A centuries-old forgotten underworld lies beneath Malta.

These lesser-known Maltese catacombs date back over 1,700 years.

The remains of a 1st-century aristocratic Roman town house featuring beautiful well-preserved mosaics.

This spectacular dome managed to survive after being bombed in World War II, in an event known as the "Miracle of Mosta."

This prison on the Maltese island of Gozo is almost 500 years old, and they have the graffiti to prove it.

This subterranean necropolis holds three floors of antler-dug grave caves.

A crumbling hospital from the 1850s on Roosevelt Island.

Great place to have a drink and a bite to eat in an atmosphere laden with New York's Bohemian past.

Haven for steampunks.

A dark, dank room in subterranean Manhattan was the site of a ghastly murder.