See massive fossils that are over 200 million years old in northeastern Arizona.

Eons ago, a rushing lava river formed this pristine passage through solid rock.

This 4,000-foot-diameter hole is touted as "the most well known, best preserved meteorite crater on Earth."

A self-built castle made by a mysterious man.

Ninety feet off the valley floor is an ancient dwelling put under protection of the U.S. government by Teddy Roosevelt.

Dramatic river bend surrounds a natural red-rocked pedestal.

Rippling sand dunes frozen in the Arizona rock.

Most-visited and most-photographed slot canyon in the American Southwest.

Two very green residential towers cleaning the air in one of Europe's most polluted cities.

A space dedicated to contemporary art and culture, co-chaired by fashion icon Miuccia Prada.

An abandoned bell tower in the middle of nowhere is the last testimony of a forgotten medieval village.

A formerly opulent Italian village is now a crumbling ghost town that no one really knows what to do with.

A brutalist gem towering over the heart of Milan.

Milan's duomo is covered in beautiful carving including a curious "dinosaur-like" beast.

An ossuary built in 1210 to collect bones from the Ospedale Maggiore of Milan.

The Emperor Tiberius' palace of debauchery.

The underground sanctuary of a mysterious ancient cult.

Architectureal fantasy quarter of Rome.

The yawning maw of this pagan visage is said to bite off the hands of liars.

An ancient Egyptian obelisk is now used to commemorate fallen Italian soldiers.

The ruins of the largest gladiatorial school in Rome are largely hidden under modern streets.

This ancient market is often referred to as the oldest shopping mall in the world.

This Roman arch marks the location of an ancient gate at one of the Seven Hills of Rome.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.