These barriers stand as stark reminders of Northern Ireland's troubled past.

Belfast's namesake river still winds below the city center, hidden from sight.

A palatial former bank building now houses what may be the poshest art venue (and former Tesco) in Belfast.

The ship built to ferry passengers and goods to the doomed Titanic is now the last remaining White Star Line vessel.

A museum marks and documents the site of the Titanic's launch.

A cave dwelling that was started by the Celts, expanded by monks, and finished by knights.

A Gothic cathedral created from volcanic rocks.

A surrealist timepiece by the artist of melting clocks.

Thousands of tourists and residents in Cologne affix "love padlocks" on the Hohenzollernbrücke Bridge to symbolize their affections.

The compass on its front was a handy tool for 17th-century merchants who needed to know the direction of the wind.

The bronzed skull of a beheaded traitor was affixed to the city gate in the 17th century.

According to a satirical conspiracy theory that persists today, this German city of 300,000 is a hoax invented by the CIA.

The last of Amsterdam's original sea level markers remaining in place.

In the days before house numbers, these colorful signs held clues about various buildings' purposes.

One of Amsterdam's narrowest houses is located right across the canal from its widest.

Three connected pink triangles memorialize persecuted gays and lesbians throughout history.

Museum preserving torture methods from the past.

The video game-themed street art first popped up in the French city.

The narrowest street in Paris.

A French artist's vague warning hangs over Belleville.

This bridge was constructed from the rubble of the infamous Bastille.

A series of abstract, silly sculptures represent the works of a great modern classical composer.

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

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.