An ancient system of freshwater caves filled with animal bones, including that of species long-extinct.

A flooded Mexican cave that hides a river beneath the water.

A swath of France so devastated by war it is still forbidden to go there.

Site of one of World War I's most horrific scenes.

This modern French ossuary contains the jumbled bones of 130,000 WWI soldiers.

Sacred waters rescued from the wrath of a volcano.

Singapore's landscaping project is home to 18 incredible Supertrees.

The largest pool at its elevation sits on top of one of the most expensive resorts in the world.

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

Where Venice's gondolas go for a tune-up.

A sinister pyramid in the Frari contains the heart of the famous neoclassical sculptor.

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

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

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

Go inside to really appreciate this 500-year-old technological masterpiece.

The inky volcanic sand leads to reefs teeming with aquatic life waiting to be discovered by snorkelers and divers.

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

The marble facade hides a little-known medical museum with early surgical instruments and rare copies of canonical texts.

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

An effigy of a trapped paratrooper dangles from a Normandy church to commemorate one remarkable D-Day event.

An unusual sand beach in the middle of a forest.

The world's largest reflecting pool is just two centimeters deep.

Abandoned blockhouses from Nazi Germany's Atlantic Wall erode into the sea on a French beach.

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.