Shakespeare, David Bowie, Dr. Who, the Saxons, Druids and Romans are all claimed to have all made their mark on these massive man made tunnels.

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

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

Wedged between two buildings at the Great Ormond Street Hospital is this secret music installation.

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

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

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

A hands-off space for graffiti artists to hone and display their skills.

Surreal riveted sea forts once protected the Kent shores from German attack.

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

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

The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

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

This abandoned racetrack looks like it is set to hold competitions in the apocalypse.

Replica of Seville, Spains famous Giralda, a former minaret converted into a bell tower.

A whimsical garden of outsider art in the Spanish countryside.

A daily pyrotechnic celebration marks a beloved tradition during Valencia's festival season.

Vibrant street art created with mixed media.

An archeological treasure trove stumbled upon by a couple of children.

The deserted remains of a luxury hotel turned hospital ward.

A completely underwater art museum sends a stark message about humans' impact on the environment.

This religious complex carved into a cliff is steeped in legends of miraculous stags and the Holy Grail.

The remains of an encyclopedic sculpture garden later used as an internment camp.

This massive fiberglass Gulliver turns every visitor into a Lilliputian.