A deadly ancient portal to the underworld.

This beautifully designed star-shaped fort was the premier defense system of the late 1500s.

An ancient and majestic fortress that played an integral role in Portugal's road to independence.

This famous city eluded concrete detection until the early 2000's.

This Tuscan lake hides a 13th century town that is still visible when the water recedes.

The second-longest continuous wall on the planet protects a hidden secret in the deserts of India.

A 12th-century fortress containing some of Berlin's oldest buildings.

A minotaur, a labyrinth, and a dubious restoration pull in visitors from around the world.

This 14th century castle withstood years of battle but is now sliding off the hills into the Danube.

The great Danube river starts here.

Historic medieval fortified city in southwest France.

The largest ice cave in the world is thought to be the entrance to hell according to Austrian legend.

An ancient artistic garden parlor full of hidden water wonders.

Stonehenge has nothing on Castlerigg’s views.

This picturesque Umbrian city has a subterranean side that has long remained hidden.

An icy lake tucked among jaw-dropping alpine panoramas.

An abandoned hotel in the highlands of Bali is shrouded in stories of ghosts, curses, and corruption.

The remains of what was, not so long ago, the world's largest city.

The ruins of an ancient island city built atop a coral reef in the South Pacific are spectacular and strange enough to have inspired H.P. Lovecraft.

This historic island prison holds one of the dungeons from the Count of Monte Cristo even though it is a work of fiction.

These stark white cliffs were spat up by the sea and are slowly being retaken by it.

Venice's island of the dead.

An isolated village literally built into the crevices and rock formations of a mountain.

The tallest bronze seated statue of the Buddha in Japan.