Picturesque ruins once held off mighty armies.

These haunting ruins belong to an ancient village that once had one hell of a view.

This Roman amphitheater is said to be King Arthur's legendary Round Table.

This lonely cast-iron structure is the only wave-washed lighthouse of its kind left in all of Britain.

The Stonehenge of Wales is one of the grandest neolithic dolmens still in existence.

The stately remains of Llywelyn the Great's mighty fortress.

A breathtaking flower tunnel bursts with beautiful golden blossoms.

Three bridges straddle a stream, one stacked upon the other.

A rocky sea serpent hides caves, blowholes, and a spectacular natural bridge.

A subterranean playground in an abandoned slate mine.

A King Kong-like gorilla hangs from the top of this commercial building in Tokyo.

In this subterranean brick arcade, a small shop continues the ancient practice of bookbinding.

The only retired U.S. nuclear missile silo that is open to the public.

These legendary stone steps were once used to haul coffins up the rockface.

This bike path traverses the treetops.

A house carved into a rock in the 1940s.

This geothermal hot spring hides beneath a slowly growing mineral dome.

An auroral research program that superheats the ionosphere.

Running under an entire mountain, the longest highway tunnel in North America is kept fresh using portal fans and jet fans.

This small city where almost everyone lives in the same building can only be reached via North America's longest tunnel.

A trading post in the North Pole where children send letters.

This frozen tunnel in central Alaska is both an engineering feat and a valuable climate classroom.

The ghostly remains of what was once the world's richest copper mine.

Gorgeous cave resemble a winter wonderland.