A marvel of Byzantine engineering under the streets of Istanbul.

This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.

A graceful natural rock formation stands astride the point where the Gulf of California meets the Pacific Ocean.

A Lovecraftian altar of nightmarish bronze chairs sits ominously on a boardwalk in Puerto Vallarta.

A flooded Mexican cave that hides a river beneath the water.

Museum on earth sciences featuring a recreation of a famous curiosity cabinet.

This city square is home to Amsterdam's oldest church and a statue honoring Dutch sex workers.

Scattered throughout London are ancient remnants of the city's former bounding wall.

One of Europe's oldest libraries.

France's national museum of scientific and industrial instruments.

The largest free-standing aquarium in the world resides in a hotel lobby in Berlin.

"There's nothing there," and other lies the KGB told us.

This endearingly discomfited urinating statue commemorates the sneaky pees had by us all.

This monument to one of the largest science fiction games of all time bears the names of hundreds of thousands of players.

Earth's tallest volcano is sacred to Native Hawaiians, and home to a dozen world-class observatories.

An unphotographable nightly march of tens of thousands of "fairy penguins" from sea to shore.

Last residence of H.P. Lovecraft featured in his story "The Haunter of the Dark."

The seawater playground of gilded-era San Francisco burned to the ground in 1966.

The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

The largest Neolithic ruins in the world hide a notably clean prehistoric metropolis.

A working steam clock, one of only a few in the world, located in Vancouver's Victorian Gastown.

A magical place that lets you play in and explore the world of Studio Ghibli.

Underground tunnels once home to crime are now back open to the public.

From Austrians to Nazis to Soviets, this historic Budapest fortress has a history as complex as the city itself.