Designed as a labyrinth, this subterranean prison was crafted without bars or doors.

An atomic bomb-proof strongbox protects the U.S. Constitution from terrorists and thieves.

An unassuming secret door once led to a legendary watering hole.

This lovely little burbling creek is actually a deadly waterway that has dragged down everyone who ever set foot in it.

Belfast's namesake river still winds below the city center, hidden from sight.

The eerie remnants of an ancient forest were hidden under the sand for centuries.

Astonishing rock formations dot the Brimham Moor landscape.

The oldest candy store in the world has been selling traditional sweets since 1827.

This strange moor has a song written about it, Bronze Age carvings, and an alien sighting.

A dramatic sculpture high up in Yorkshire with vistas on an industrial scale.

This pyramid in southern Sweden houses the remains of an eccentric nobleman, who spent his life doing whatever he felt like doing.

The ruins of a monastery where Saint Bridget of Sweden had her revelations recorded.

This seventh-century fortification marked the western border of the Lombard duchy of Spoleto.

This enormous sinkhole hides a secret world created from divine wrath.

The enormous 250-room Vanderbilt mansion conceals hidden doors and secret passageways.

This "living building" on Georgia Tech's campus is home to a blueberry orchard, honeybees, and more than 900 solar panels.

Thousands of American timepieces on display in the former clock-making capital of the country.

This petite park lets visitors see and touch examples of fossilized dinosaur tracks.

More than a century of radio history is told in these displays.

A vast collection of rare books, manuscripts & extraordinary objects illustrating the history of medicine.

The oldest place in Hartford is a cemetery with thousands of bodies stacked beneath the grass.

A quirky assortment of musical marvels from around the world.

The mysterious headstone of an unidentified bank robber was seen visited, every year, by a woman in black.

The headquarters of the famous Yale society.