Ancient ruins hidden underneath a modern luxury hotel.

A mysterious chasm in the heart of the ancient Roman Forum, the Lacus Curtius was once believed to be a gateway to hell.

The yawning maw of this pagan visage is said to bite off the hands of liars.

The pockmarks across the building's facade are remnants of the Blitz.

Legend says these colorful, invasive residents are the descendants of birds released by Jimi Hendrix.

An architectural oddity hides in plain sight on this iconic London bridge.

This one has sailed five times as far as Sir Francis Drake's famous ship.

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

Lovers proclaim their unbreakable bond by leaving their names on locks fastened onto the Shoreditch fence in East London.

Giant, friendly pelicans in Central London, the most unlikely of places.

Eighty million natural history specimens call this gargantuan museum home.

Rebuilt remains of a temple to Roman god Mithras.

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

The popularity of Sherlock Holmes led to the creation of his fictional address, turning the orderly London street numbers askew.

This series of Baroque passageways in Vienna's cultural center mix old architecture with modern art.

Vienna's giant wonder wheel had a starring role in one of the greatest noir movies of all time.

Op-art architecture designed for happiness.

A spherical "micro-nation" in the heart of Vienna.

The former private quarters and office of Dr. Sigmund Freud now hosts a museum housing the works of the founding father of psychoanalysis.

Built to give thanks for the passing of a plague epidemic, this cathedral is a one-of-a-kind architectural hybrid.

The topiary maze at the Habsburg hunting lodge.

The final resting place for the remains of 143 Habsburg royalty.

Central Europe's largest collection of mini-bottles is on display at this museum devoted to a bitter Hungarian liquor.

Once the home of a hermetic monk, this cave now houses a small church.