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.

This logic-defying 16th-century Tudor manor still stands to the delight and astonishment of onlookers.

This macabre monument atop an ancient well gives a nod to a brutal episode in Scottish clan history.

Scotland's only funicular railway is also the United Kingdom's highest.

Human-made caves excavated to hide a secret society of wealthy pagans.

An engineering error ensured this cavernous 350-foot-long subterranean thoroughfare was never used as intended.

Colin Stokes "got a bit carried away" and built a castle that looks like something out of Tolkien.

This magical museum celebrates the life and work of a seminal children's book writer (and occasional spy).

During this Shetland Islands celebration, hundreds of torch-carrying "guizers" lead a procession to burn a viking longboat.

The highest peak in Wales is said to be the final resting place of a giant killed by King Arthur.

Long abandoned underground complex to keep royals safe from a nuclear war that never came.

One of Scotland's most important modern structures, it just made the '100 Most Endangered Sites' list.

This foolish Victorian sea fort was converted to a deluxe hotel.

This short bridge is the only thing connecting a tiny Irish island to the mainland.

A red sandstone freestanding monolith rises out of the sea in northern Scotland.

A familiar otherworldly creature guards this 12th-century Scottish abbey.

A house of bones — but nobody is quite sure to whom they belong.

This picturesque view of the Scottish Borders was beloved by Sir Walter Scott.

A hermit's tiny cell built into the cliffside that saved him in South Pembrokeshire.

On this stunning Scottish island, you can walk in the footsteps of dinosaurs.

A miniature Italianate fantasy village on the coast of Wales and the set of ITC classic "The Prisoner."

The mysterious megaliths have enchanted and intrigued visitors for thousands of years.

A 200-year-old prison that now houses a peculiar crime museum.

This abandoned asylum in Wales might look haunted, but it's still beautiful.