The largest Art Nouveau complex in the world is a Barcelona hospital with a 600-year-long history.

A beautiful but toxic fountain of mercury.

Government testing at the asylum briefly explored using marijuana as a "truth serum" on Nazi prisoners of war.

A giant interactive periodic table with real samples of the elements.

Underneath the city that invented pilsners are a series of historic tunnels that have been storing beer for centuries.

A museum of alchemy and a basement laboratory all housed in an ancient building in a Gothic tower.

A Jenga-style cylindrical tower of books that appears to go on forever.

The hidden lair of an alchemist rediscovered in a 2002 flood.

This replica of the Greek structure is in the heart of Nashville.

For two weeks every June, fireflies in the Smoky Mountains perform an astonishing light show.

Better known as the "Body Farm," this unique research facility has been studying how human bodies break down since the 1980s.

The only remaining Cold War–era Minuteman II missile control center in Missouri.

The first rural cemetery west of the Mississippi is home to the elaborate tombs of St. Louis' local beer barons.

The oldest bar in Missouri is hidden within a limestone brewery cellar more than 50 feet underground.

The history of the treatment of mental illness, illustrated in all its gory detail, and housed in an asylum.

A waterfall 600 ft. inside a mountain, once the bane of gold miners, is now a spectacle of nature.

Dedicated to the history of the 1,000 nuclear missiles that once covered the Great Plains.

South Dakota's oldest operating brewery is housed inside a historic firehouse.

Thanks to a loophole in property ownership, this alleyway has become a cacophonous street gallery.

Hidden amid a collection of trails is the site of a tragic crash.

The lesser-known lost colony of Roanoke was a safe haven for formerly enslaved people during the Civil War.

The time thousands of bags of chips washed ashore in the Outer Banks is memorialized in a local museum.

Conjoined twins Daisy and Violet Hilton were stars of the vaudeville circuit.

This rare basilica nestled right in downtown Asheville is an architectural marvel, inside and out.