Once the tallest building in the world, this abandoned church is now a monument to the destruction of WWII.

A favorite Paris bar for U.S. expats, including Gershwin, Hemingway, and James Bond.

The spiral staircase hidden inside this giant Parisian column was likely built for Catherine de Medici's astrologer.

This 400-year-old tree is actually an immigrant from North America.

A simple plaque on the facade is the only reminder that the treaty ending the American Revolution was signed inside.

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

Twelve grave-faced marble angels guard the emperor's tomb at Les Invalides.

This off-limits room in the National Library of France is home to a system of pneumatic tubes, once used to bring books to readers.

The bones of the Biblical Magi traveled to Constantinople and Milan before landing in their current home inside an elaborate golden reliquary in the Cologne Cathedral.

This small condiment museum makes all manner of mustards with a 200-year-old mill.

The last bastion of Roman presence north of the Alps looms over a German town.

The birthplace of Karl Marx is now a museum dedicated to the communist revolutionary's life and work.

This natural "sea of stones" was once used by the Romans as a quarry.

A larger-than-life monument to the inventor of the Bunsen burner.

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.

This 17th-century street hidden beneath Edinburgh was 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.