An accidentally hidden library containing the ephemeral history of New York.

This Manhattan skyscraper is a reminder of the brief but glorious age of urban air taxis.

This translucent sculpture represents a quintessential component of New York City's skyline.

Pulled from the rubble of one of the worst modern tragedies, a steel crossbeam became a symbol of hope for New Yorkers.

The first piece of public art dedicated to LGBT rights.

This historic building housed the surviving crew members of the Titanic, who were left destitute after the wreck.

This historic address is home to the beloved ball and is an almost totally empty building among the most expensive real estate in the world.

A bit of retro transportation flair preserved in the world's largest department store.

This sculpture by artist Fritz Keonig survived the 9/11 attacks and now stands as a monument to the victims.

This mosaic dedicated to John Lennon was tended for years by a Beatles super-fan.

Whisper secrets in a busy station only to be heard on the other side of this acoustic arch.

A trough of 700 strange bronze faces runs through a narrow alley in Ljubljana's old town.

This abandoned Slovenian military barracks has become a punky-political art squat.

This building has two facades of clashing architectural designs.

Sofia's legislative center is continuously reinventing itself.

"Holy Wisdom" overlooks the city that became its namesake.

A vast Communist-era exhibition space in the Bulgarian capital.

The second-largest Jewish temple in Bulgaria was deserted after World War II, seized by the communist government and then abandoned again.

A striking symbol of the Bulgarian capital.

Nine centuries-old fortresses guard the walls of this medieval Romanian city.

This communist behemoth is the heaviest building in the world and a legacy of a brutal regime.

Something's watching you.

Once an opulent Art Nouveau hotspot, the Constanța Casino is one of Romania's most beautiful abandoned buildings.

The possible imprisonment of Vlad the Impaler here earned the medieval castle its nickname, "Dracula's Castle."