A hidden tunnel leads to an abandoned slate quarry that's now a secret swimming spot guarded by cliffs.

One of the world's oldest trees comes with a Welsh legend intertwining it with a spirit prophesying death.

19th century tunnels burrowed under Liverpool.

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

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

A 550-square-meter micro-nation off the coast of Britain.

The biggest and deepest Cold War bunker open to the public can be found in southeast England.

A 4,500-year-old forest is slowly being revealed from beneath the sands of a Welsh beach.

A harem-inspired Moorish fantasy design in a Victorian-era Welsh castle.

This well-preserved Cistercian Abbey in a romantic pastoral setting has inspired artists like the poet Wordsworth.

In Twickenham sits a small whitewashed castle that inspired Mary Shelley's 'Frankenstein.'

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

Western Europe's only representation of a Roman goddess still in its original location sits in an unassuming park.

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

This beautiful "lagoon" is actually a toxic soup filled with cars, carcasses, and trash. When warnings couldn't stop people from going for a dip, the town dyed it black.

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

The largest cave chamber in Britain—big enough to fit a whole cathedral inside.

The stately remains of Llywelyn the Great's mighty fortress.

Fantastically decorated underground chambers built in the 1760s.

Three bridges straddle a stream, one stacked upon the other.

A rocky sea serpent hides caves, blowholes, and a spectacular natural bridge.

Giant, friendly pelicans in Central London, the most unlikely of places.

A subterranean playground in an abandoned slate mine.

The decaying ruins of Wales' first asylum for the mentally ill.