Rebuilt remains of a temple to Roman god Mithras.

The commemorative stone column conceals a secret laboratory.

Tucked away in the Tower of London is the weapon once used to execute high-profile prisoners.

On October 17, 1814, a 15-ft. wave of beer ran rampant through the streets of London.

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

A "Texanic" (gigantic Texas) collection of Art Deco architecture at Dallas' historic Fair Park.

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

The heads of two giant mythological water horses honor Scotland's horselord past.

The 2,000-year-old Roman columns are hidden within Barcelona's Gothic Quarter.

Three columns of volcanic rock shoot out of the ocean.

If a glacial river suddenly diving into a sheer chasm via a dramatic, multi-stage waterfall isn't peak Iceland, it's certainly close.

This valley of hot springs and boiling mud pots is home to the record-holding "Geysir," which originated the English word.

Iceland's largest lagoon is home to stunning multi-colored icebergs.

This fantastical waterfall is hidden in a cave located in a cracked cliff.

Medicinal spa created with the discharge from a geothermal energy plant.

The first tunnel to be dug beneath a river, repurposed as an art and museum space.

A 2000-year-old Roman amphitheater lies just below Guildhall Yard.

The handiwork of a underdog clockmaker who solved an impossible problem for an Empire, and forever revolutionized seafaring.

A unique tribute to the millions of animals who served and died for the British military.

The watery entrance for condemned prisoners heading to the Tower of London is still visible along the Thames.

The barrel-made lookout from Shackleton's final ship is tucked away in the crypt of one of London's oldest churches.

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

This legendary stone is of unknown origin, and throughout history has been a literal touchstone for British leaders.

Commonly acknowledged as the only public monument to the Devil himself.