A subterranean playground in an abandoned slate mine.

The one-time stronghold of the noble Douglas family, now home to ghost sightings and nesting birds.

The ruins of this gorgeous clifftop castle look like something from a fantasy novel.

A unique waterfall that some believe attracts the fairfolk.

The largest greenhouse in the world is home to over one million types of plants.

A makeshift shrine to the late rock star is still kept up on the nondescript façade outside 1 Logan Place.

400-year-old garden, restored to beauty.

Ancient cliffs resembling a kilt on Scotland's second largest island emit an entrancing sound.

A charming 17th-century alley is lined with secondhand bookstores and antiquarian shops.

The largest of London's subterranean rivers and once a mad, bad center of London life.

The world's first underground farm is in a World War II bomb shelter beneath the streets of London.

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

Catacombs turned marketplace.

The signs of a long buried road can still be found hidden beneath a London sewer grate.

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

A island reachable by a causeway at low tide is possibly one of the earliest Western European locations to be identified in text.

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

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

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

An unusual round church in London with a Templar past.

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

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

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

17th century streets hidden under Edinburgh were once a breeding ground for the black death.