A lush indoor jungle fills this bustling Madrid transportation hub.

The largest of Denmark's seven mysterious round churches stands on an island in the Baltic Sea.

One of the few remaining ancient aqueducts described by Frontinus as 'the most solemn testimony of the Empire.'

These magnificent city walls rise from the barren sun-baked Spanish plains like something straight out of a fairytale.

This lucky bronze frog was gifted to Madrid by a casino.

One of the world's oldest and most massive living organisms is a grove of quaking aspens.

This walkway is so narrow it needs its own light to stop pedestrian traffic jams.

Legend says if you stick your thumb in the hole and it gets wet your wish will be granted.

Ninety feet off the valley floor is an ancient dwelling put under protection of the U.S. government by Teddy Roosevelt.

For six months of the year, three giant mirrors direct a pool of sunshine onto this otherwise occluded town.

Commonly referred to as the most photographed Victorian home in the United States.

An abandoned network of military bunkers and tunnels open for anyone willing to descend into the darkness.

This irregularly shaped sea fortress has defended not one but three different sovereign states.

Colorado Boulevard’s historic bookstore, operating since the bustling city of Pasadena was merely some orange groves and a few ostrich farms.

"the sweltering inhabitants of Charleston and New Orleans, of Madras and Bombay and Calcutta, drink at my well . . . The pure Walden water is mingled with the sacred water of the Ganges."

This iconic L.A. bookshop is housed in an abandoned bank—both symbolic and chic.

Sweden's "good old days" painstakingly re-created in a model village.

It houses the remains of a 17th-century version of the Titanic.

Stockholm's decidedly unique public transit system has doubled as the world's longest art gallery for decades.

On the Oregon coast there is a natural hole that seems to be draining the sea.

This 1967 experiment in modular architecture was designed to be a new model for urban living.

A giant toy jack made of numerous welded-together fire hydrants.

Secret Soviet city once home to Russian cosmonauts and space training facilities.

At just 51 feet and 10 inches long, it's the shortest street in England.