Medicinal spa created with the discharge from a geothermal energy plant.

The intricate stonework of Machu Picchu serves as a testament to Incan engineers.

This elaborate towering structure was once famed for its revolutionary height.

Ant sculptures march up the side of what used to be an iconic spot in punk rock lore.

Yo, Adrian!

One artist's love letter to his city and its citizens plays out across dozens of bright murals painted on the sides of decrepit buildings.

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

This little-known collection holds everything from "Dracula" notes, to the manuscript of "Ulysses," to the forged works of Shakespeare.

Huge masonic hall in Philadelphia with lavishly themed rooms.

Mural made of 100,000 pieces of hand blown glass, and until recently, the largest glass mural in the US.

A layered tribute to the work of mosaicist Isaiah Zagar.

World's first "penitentiary," meant to be humane, drove men insane.

The celebrated eyesore created by starry-eyed sweethearts.

A 16th-century fountain marks the site of a lost medieval cemetery once filled with mass graves.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

The marble facade hides a little-known medical museum with early surgical instruments and rare copies of canonical texts.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

Ancient serpentine sacrifice holder that has stood the test of time.

This remarkably well-preserved Egyptian plinth is pretty well traveled for a giant piece of stone.

A small etching on the white marble parapet was written in runic script by a Viking mercenary.

Five sultans and their families rest in eternal splendor, teeming with overlooked beauty and drama.

Legend says if you stick your thumb in the hole and it gets wet your wish will be granted.

A marvel of Byzantine engineering under the streets of Istanbul.

This little Greek plateau is home to thousands of rustic, white-sailed windmills.