What looks like a normal building is really a hollow facade hiding a subway vent.

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

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

France's most famous cemetery, with some of its most curious tombs.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

In the past, Renaissance sex workers were encouraged to entice customers on this bridge.

This deconsecrated church has seen many uses, from a tobacco storehouse to a cinema.

The pile of books behind the dignified writer gave the statue a most undignified nickname.

This Roman statue is missing a limb from when the Venetians stole it from Constantinople during the Crusades.

The only museum with its curator—and her pets—buried inside.

Thousand-year-old Viking runes adorn an ancient Greek lion at the entrance to the Venetian Arsenal.

Two stone columns flank the grand Venice square, but there were supposed to be three.

An extensive collection of reliquaries containing the bones of saints housed in the largest Catholic church in Venice.

100 year tradition of fist fights, marked by marble footsteps.

This palatial spiral staircase was so unusual the name "snail" was given to the noble family who built it.

This Venice bookstore has resigned itself to constant flooding by keeping its books in bathtubs and boats.

Legend has it the famous Robin Hood would attend mass at this 12th-century Cistercian abbey in Sherwood Forrest.

One of the earliest large-scale "hyper" roofs in the U.K. is now a derelict roadside attraction.

The largest oak in England is said to have been the hideout of Robin Hood and his Merry Men.

When methane gas started seeping up from the ground, they picked up and moved the entire town.

This restored 17th century "conduit house," often mistaken for a watchtower, actually supplied water to nearby Bolsover Castle.

A gorge full of caverns where early humans lived alongside cave hyenas, giant mammoths and wooly rhinoceroses.

This centuries-old canal known as the Cuckoo Dyke has a staircase of locks suitable for a giant.