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.

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

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.

The former home of Samuel Clemens and family remembers the happiest period of the author's life.

Small monoliths of several varieties of stone sit alongside a wooded path in the Bronx.

This massive World War II calculator hearkens back to the days when "computer" was a job title.

This peaceful Massachusetts graveyard was one of the first "garden-style" cemeteries in America.

This parking garage was the site of what was - at the time - the largest cash robbery in history.

The site where Charles Dickens gave his first public reading of "A Christmas Carol" in the US.