Once America's longest-manned military fort, now an abandoned hulk.

The third-tallest lighthouse in the United States stands as a reminder of Atlantic City's early days.

"A hideous albatross in the Hudson River Valley."

A well-preserved example of an early industrial operation in the northeastern United States.

This charming colonial alleyway is one of the oldest continuously used residential streets in the U.S.

The meeting of the northwestern corner of Delaware, the northeastern corner of Maryland, and the southern edge of Pennsylvania.

This sinkhole in the Arizona desert is filled with carbonated, arsenic-rich water

A piece of telecommunications history.

If you look closely at the oldest bridge in Paris, you might notice hundreds of ugly faces leering from its sides.

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

19th century pendulum and a clock restored by a rogue group of guerilla artists.

One of Jeff Koons’s largest sculptures and his first commemorative work.

A simple plaque on the facade is the only reminder that the treaty ending the American Revolution was signed inside.

This off-limits room in the National Library of France is home to a system of pneumatic tubes, once used to bring books to readers.

This former “ante-chamber to the guillotine” has been refurbished to illustrate the brutal history of French justice.

Twelve grave-faced marble angels guard the emperor's tomb at Les Invalides.

The water-powered carriages are long gone, but for over a century this funicular railway has been the fastest route to the highest point in Paris.

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.

This massive library holds what was once the largest book collection in the world.

Every year model trains zip through a miniature New York City that's now 150 landmarks strong.

One of America's founding fathers lived in this London lodging between 1757 and 1775.

The oldest operating apothecary in the US has treated everyone from Thomas Edison to Mark Twain.

Because holiday cheer is at its finest when it includes eerie mannequins, right?

The home of one of America's most wealthy industrialists is now a historic site that remembers a more decadent time in the country's history.