A historic London pub with prison cells in the cellar and a "spy room" on the second floor.

Famously misunderstood wonder of the ancient world

A 1,215-foot tunnel transports pedestrians beneath the River Thames.

The only university zoological museum in London houses extinct animals, bizarre natural history specimens, and a Micrarium of microscopic creatures.

One part local history, 99 parts gruesome haunted house.

A subterranean ice warehouse that has lain hidden beneath central London for over a century is now open to the public.

Winding paths and ramshackle mausoleums in the oldest "park cemetery" in London.

One of the few remaining casualties of the London Blitz, this destroyed church has become an enchanting public garden.

London's creepiest cemetery was once the site of dueling magicians and mobs of stake-carrying vampire hunters.

A stunning medieval castle owned and restored by the famed actor Jeremy Irons.

A massive Celtic cross secretly growing in an Irish forest.

An ancient holy well said to be used by the patron saint of sailors and travelers, lost in the boggy west of Valentia Island.

When the tide is high, you can see the Atlantic pushing up through a deep, jagged hole in the rock.

The severed head of a 17th-century Irish martyr lies within an intricate golden shrine.

This 5,300-year-old tomb with mysterious carvings crowns an Irish mountain.

The block of aged dairy is probably still edible after 2,300 years in a peat bog.

This courtly monument celebrates an influential comedian who turned taking the piss out of the powers-that-be into an art.

This 30-foot pile of rocks may be the grave of a mythological Irish queen.

The ruins of an 800-year-old monastery boasts the tallest round tower in Ireland.

This controversial Irish show cave is home to a massive formation known as the Great Stalactite.

The Grand Lodge of Ireland is one of the oldest in the world, full of fantastical, elaborately themed rooms.

A stunning Victorian abbey and walled garden on a lakefront.

This singular Irish dolmen is more Frank Lloyd Wright than upright stonehenge.

A spectacular hidden beach in Ireland has the dark, disturbing nickname of "Murder Hole Beach."