Apartment complexes built into huge repurposed natural gas tanks.

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

In this historic region, homes and churches are carved into ancient volcanic rock formations.

A deadly ancient portal to the underworld.

Roman ruins top white travertine terraces formed by ancient hot springs.

A marvel of Byzantine engineering under the streets of Istanbul.

Ozymandias’ time-keeping temple.

Kings Cross Station pays tribute to its role in Britain's best-selling book series.

A six-ton wedding present for the King of the Scots.

World's strongest whirlpool.

The convergence of two currents makes this a surprisingly pleasant getaway in the Arctic Circle.

Of course Salvador Dalí lived in a castle...

A darkly erotic open-air statuary in Catalonia.

Medieval village and microminiature museum.

Wonderland replication in progress by one persistent man.

Village situated on a narrow volcanic escarpment.

Religious dwelling in a Spanish volcano.

Famed architect Antoni Gaudi built this generous thank you gift to a friendly host who housed him during a job.

A beautifully eclectic mix of architectural styles.

The Pope’s secret escape route.

Rome’s legendary peephole vista is also a former Crusader stronghold.

The skull of the patron saint of lovers lies in the Basilica di Santa Maria in Cosmedin—maybe.

Pilgrimage site for star-crossed lovers.

Site of a classic 19th-century physics experiment to prove the earth is rotating.