This massive subterranean fissure is known as the "underground Grand Canyon."

A whimsical bridge adorned with decorative dragons.

Castle and secret tunnel withstood a siege until its owner was killed by the Holy Roman Empire, while sitting on the toilet.

This one-time health resort was used by aristocrats around the world.

The ominously named cave is home to some remarkable ancient art.

This was once the center of the Birdman cult, and is now a World Heritage Site.

The largest stone moai on Rapa Nui.

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

A spooky juxtaposition of caves and graves gives this legend-filled cemetery a particularly haunting vibe.

A gorge full of caverns where early humans lived alongside cave hyenas, giant mammoths and wooly rhinoceroses.

A spiral staircase connects the limestone cave with Pembroke Castle, birthplace of King Henry VII.

An 18th century monument regularly tagged with unsettling graffiti referencing a 1940s unsolved murder.

A mysterious bridge over a sunken Roman road.

The ruins of this medieval castle in Northern Ireland have inspired artists from C.S. Lewis to Led Zeppelin.

Under a church hides a forgotten 13th-century ossuary containing the remains of around 2,500 people.

Near Birmingham, England three and a half miles of top-secret tunnels housed a World War II “shadow factory” and a Cold War nuclear bunker.

There's a labyrinth of medieval chambers hidden deep below a 15th-century pub.

This Roman amphitheater is said to be King Arthur's legendary Round Table.

One family's environment-friendly hobbit-house in Wales.

Fantastically decorated underground chambers built in the 1760s.

This romantic, decaying 13th-century ruin on the edge of Sherwood Forest is reputedly one of England's most haunted places.

A beautiful ruin of a 14th-century Plantagenet castle built by a medieval mercenary.

Although it appears ancient and mysterious, the quirky folly was actually built 200 years ago by an eccentric estate owner.

High Victorian fantasy created a fairytale castle out of a ruined fortress in Wales.