A 140-year-old castle with underground caves hidden in San Francisco.

These remote Florida islands have a history of sea turtles and sunken treasures, and one of the world's largest coastal brick fortresses.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

A theme park that suffered massive flooding during Katrina was abandoned, and now proves irresistible to trespassing explorers.

This secluded aqua waterfall in the Grand Canyon is the perfect swimming hole, and the Havasupai tribe's fiercely protected natural wonder.

Dramatic river bend surrounds a natural red-rocked pedestal.

Wondrous sandstone hoodoos where Native Americans collected clay for pottery, 9,000 years ago.

This jaw-dropping 19th-century bridge uses its reflection to form what appears to be a perfect circle.

On Japan's Tashirojima Island, cats are king.

An immersive environment that is part haunted house, part choose-your-own-adventure, and part jungle gym.

The mysterious, fantastical woodland inspiration for "The Lord of the Rings."

An eerie, derelict amusement park in Berlin.

An exquisite tunnel of cascading flowers.

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

Most-visited and most-photographed slot canyon in the American Southwest.

The macabre collection of Jacques Sirgent, an expert scholar on the undead and all of their trappings.

World's first "penitentiary," meant to be humane, drove men insane.

A collision of human error and natural geothermal pressure created this rainbow-colored geologic wonder.

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

Rippling sand dunes frozen in the Arizona rock.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

World's largest collection of serial killer artwork and other macabre exhibits.

An artsy hidden staircase leads to breathtaking views of San Francisco.

At one time this small mining village was the highest town in America but now it sits abandoned while the mine churns on.