The one-time stronghold of the noble Douglas family, now home to ghost sightings and nesting birds.

At high tide this strange footbridge appears to have no purpose whatsoever.

Ruined castle with subterranean vaulted Goblin Ha' or "Goblin Hall."

Its dark depths conceal millions of rare, delicate crystal formations.

This Roman quarry once held secret masses and smuggler's contraband and is now home to rare species of bats.

Hidden in a grotto 26 feet below Nottingham's oldest hotel, this stylish subterranean drinking den is notoriously hard to find.

This fascinating cave is packed with prehistoric rock art and Stone Age tools.

Accessible only at low tide, the deserted island is dotted with World War II ruins.

This allegedly haunted 16th-century tower house-turned-museum exhibits thousands of spectacular objects.

A hidden tunnel leads to a medieval queen's shrine buried beneath a parking lot.

The awe-inspiring ship from the golden era of cruising has an incomparable history and a reputation as one of the world’s most haunted places.

This abandoned, windowless concrete block is one of the strangest buildings in Tulsa.

A golden egg-shaped sauna in the Arctic tundra.

The final resting place of the controversial chess champion.

Iceland's "Smoky Valley" is a beautiful river that is shrouded in a constant cover of steam clouds.

This eye-popping Icelandic crater lake is surrounded by red volcanic rock.

The now-abandoned tunnels were built for the transcontinental railroad on the route where the first wagon train entered California.

An abandoned railroad tunnel forgotten by time awaits inquisitive hikers.

The abandoned funicular tunnel was a secret base for the BBC during World War II.

Moss and stone turn this waterfall into an otherworldly liquid veil.

This fairy-tale castle would make Dracula (who is said to have been imprisoned there) jealous.

One of the oldest churches in Romania is believed to have been built on the site of a pagan temple.

The romantic botanic tunnel feels like a well-kept secret.

The abandoned ruins of a beautiful medieval castle just outside Barcelona.