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.

A labyrinthine underworld of tunnels and caverns hides beneath the city.

This Scottish manor house is a lush relic from the life of a literary great.

One of the few standing Roman lighthouses and a labyrinth of tunnels are a couple of the secrets at this historic castle.

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

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

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

This dreamy street will transport you to wintertime in Victorian England.

Overlooking a Scottish loch, this beautifully crumbling castle once inspired a poem by William Wordsworth.

This Gothic Revival castle used as a shooting location for "The Secret Garden" and other films.

The oldest residential street in Europe also features an optical illusion.

The only fixed chain and post trail in Scotland snakes its way along the base of a towering seaside cliff.

St. Mary's Asylum was isolated and abandoned before being converted into a Bed and Breakfast.

These ruins witnessed the final gathering of the failed Jacobite Rebellion.

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

This one has sailed five times as far as Sir Francis Drake's famous ship.

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

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

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

Below a historic English market town lies a secret underground city complete with kitchens, laundries, storerooms, and an underground lake.

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

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

A castle in the Scottish Highlands has been continuously inhabited since the 1300s.

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