This Luxembourg cave is covered in cryptic looking marks left by mining activity.

A small etching on the white marble parapet was written in runic script by a Viking mercenary.

This 6th-century church has the largest and best-preserved collection of Byzantine mosaics outside Constantinople.

An oddly conical beacon perched atop beautiful Irish cliffs.

Thousands of flowers create enchanting pink tunnels over the city streets.

This early version of the guillotine was used to execute the man responsible for its construction.

Home to the world’s oldest seismograph and a four-ton steel ball that makes its own earthquakes.

The oldest restaurant in the world is this old-world Spanish eatery, still known for its suckling pig.

One of the few remaining ancient aqueducts described by Frontinus as 'the most solemn testimony of the Empire.'

This underground church is topped with a 500-foot cross that is seen by many as a beacon of fascist hubris.

This Roman statue is missing a limb from when the Venetians stole it from Constantinople during the Crusades.

The oldest housing complex in Riga is a mini timeline of architectural trends.

The Tudor-style cottage in the middle of the square is not quite what it seems.

The Metropolitan Police refurbished the blue box (perhaps not coincidentally) the same year "Doctor Who" returned to TV screens.

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

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

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

A tiny chapel clings to a precipice above Germany’s gemstone capital.

The palace window through which Protestant nobles flung two Catholic regents, thus sparking the Thirty Years' War.

This palatial spiral staircase was so unusual the name "snail" was given to the noble family who built it.

The remains of the oldest synagogue in Britain lay forgotten for seven centuries, hidden under a humble shop.

A copy of the famous statue can be found in this Florence piazza, along with one of the most breathtaking views of the city below.

Now harmless, this looming post struck fear into the heart of medieval troublemakers.

A former ironworks that was the first UNESCO industrial site.