The longest continuous lava tube in the continental U.S. offers 2.5 miles of dark explorations for the adventurous.

An isolated dive site teeming with cnidarians.

A gargantuan crystal kingdom.

A museum devoted to bringing to light forgotten or neglected histories through exhibitions, education and public programming.

The history of occult science is turned into a creepshow at this lurid Prague tourist attraction.

An eerie, derelict amusement park in Berlin.

An ancient capital city that dates back 3,000 years old and harbors many surprises.

The ghost town left by the worst nuclear disaster of all time is being taken over by nature and urban explorers.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

An exquisite tunnel of cascading flowers.

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

A beautiful and abandoned New York subway station from 1904, complete with chandelier.

This cuddly rodent has a place of honor at a Russian institute for genetic research.

This unauthorized stone mermaid was secretly carved out of public stone before being permanently preserved.

Mysterious 500-year-old tapestries depict a unicorn hunt.

Dodge prohibitionists and slide through the telephone booth in Crif Dogs.

Plot your trip through the city on the sidewalk.

Tucked away amidst some of the most famous addresses in the world is New York's only fraction of a street.

Amid the bustle and noise of the Financial District hides a secluded garden oasis above the city streets.