A crumbling hospital from the 1850s on Roosevelt Island.

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

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

Hidden above a drug store on a nondescript corner of Lincoln Heights is a private clubhouse for intrepid explorers.

A rotating cast of guys and dolls in front of a Washington, D.C. building.

A student driver's nightmare.

The popularity of Sherlock Holmes led to the creation of his fictional address, turning the orderly London street numbers askew.

Skeletons dressed in extravagant 18th century royal costumes cavort in the aisles of this German basilica.

This centuries-old mine remained hidden beneath a hotel bar until 2013.

In a 345-million-year-old cave that's 200 feet underground, dinner awaits.

Giant, natural amphitheaters made of delicate geological formations called "hoodoos."

A castle that was built stone by stone by an eccentric journalist.

The world's third-largest crystal sphere has sat in the Penn Museum since 1927 with the exception of one outlandish theft.

Home to several unusual artifacts, including a huge painting of George Washington.

The spectacular collection of one of the world's best woodcarvers.

A slab of sandstone covered with mysterious 1,000-year-old rock carvings from a vanished North American culture.

Unique collection containing a wide and curious range of contraceptive items.

This spectacular public market has got meat, fish, vegetables, baked goods, and enough kielbasa for a lifetime.