This series of Scottish caves has no clear purpose other than baffling historians.

A fantastic castle rumored to be the birthplace of King Arthur.

"Respectful to the dead, safe and sanitary to the living, dedicated to the Genius of Memory..."

This castle was established by the 4th Duke of Montrose and is connected to the Buchanan Clan.

One of the oldest scientific gardens in Britain, containing more than 13,302 plant species in its core collection.

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

Amazing and mysterious Neolithic settlement on Scotland's Orkney Islands.

Rebuilt remains of a temple to Roman god Mithras.

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

Northern Ireland's Brobdingnagian stepping stones.

A 30-acre garden inspired by the principles of modern physics.

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

Hidden beneath the streets of London is a secret subway that only carried mail.

This collection of illusory paraphernalia has been culled from the history of London's premier magician's club.

Kings Cross Station pays tribute to its role in Britain's best-selling book series.

The sign at the garden gate reads: "These Plants Can Kill."

This churchyard arbor is surrounded by hundreds of gravestones placed there by author Thomas Hardy.

The mysterious, fantastical woodland inspiration for "The Lord of the Rings."

These fantastically named natural pools look like something out of a Scottish myth.

A kaleidoscopic warehouse-maze of handmade neon signs.

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

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

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

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.