A subterranean playground in an abandoned slate mine.

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

Ruined 11th century abbey built by King David I in Edinburgh, Scotland.

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

This allegedly haunted 16th-century tower house-turned-museum exhibits thousands of spectacular objects.

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

A book bound in the skin of an infamous criminal, along with countless other specimens.

The fountain marks the area where hundreds of accused witches were burned at the stake.

360° views of the city plus five floors of optical illusions and hands-on science.

The music is over at the Ballhaus Riviera Ballroom, but the ballroom remains in haunting disrepair.

The world's first clock to use set theory as a measure of time required the invention of a new light bulb just to maintain its obtuse measurements.

A wunderkammer of mysterious objects and stories.

Home to 10,000 medical specimens including the remains of Rudolf Virchow's extensive collection of pathological specimens.

A museum dedicated to a Cold War era sausage.

What started as a tongue-in-cheek shrine is now a full-blown museum celebrating "The Hoff" and his longtime love affair with Berlin.

Cold war icon and onetime "hole" in the iron curtain.

A tour of underground Berlin takes you through WWII air raid shelters and tunnels used to smuggle people out of East Germany.

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

Mary Kelly was last seen here before she was murdered by Jack the Ripper.

This pump on the bad side of Victorian London betrayed a horrifying revelation about the water supply.

Richard III supposedly disposed of his nephews' bodies here in an effort to seal his claim to the throne.

A famous burial ground with a history both grim and literary.

A sculpture outside London's oldest cathedral honors the Mohegan chief who died while petitioning the king.

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