Enormous lake at the bottom of a unique cave system.

The secret city built by the Manhattan Project.

A mysterious inscription may hold a clue about the "Lost Colony" of Roanoke.

A 300-yard long natural tunnel runs through a ridge deep in the Appalachian Mountains.

A mysterious monument meant to be a guide into "an Age of Reason" was destroyed after an apparent bombing.

One man’s religious convictions led him to build this stone castle meant to withstand an apocalypse.

Abandoned for 60 years, this submerged bridge was completely forgotten until the lake was drained.

The highest point in Tennessee, Clingmans Dome bears witness to the ravages of one type of insect.

This road in the Great Smoky Mountains was supposed to assuage a displaced community, but ended up a $52 million dead end.

The remains of the iconic train crash from the movie The Fugitive can still be found rusting along the Great Smoky Railroad.

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

An annual celebration of the region's successful elk reintroduction program.

A science fiction-inspired apartment building designed by a famous Spanish architect.

Moorish signal station on a stratgic rocky outcrop.

The chefs at El Diablo use heat generated by an active volcano to cook the food.

This magical space partly designed by Antoni Gaudi is one of the best examples of romantic gardening in the Mediterranean.

Of course Salvador Dalí lived in a castle...

This massive fiberglass Gulliver turns every visitor into a Lilliputian.

A charming Spanish village sits atop this natural limestone overpass.

The earliest known map to show America, discovered by accident at a Paris bookshop.

The abandoned ruins of a beautiful medieval castle just outside Barcelona.

Stunning Roman aqueduct used for 1900 years and still standing.

Considered the birthplace of the Copper Age and Bronze Age, the river is tinted red from 5,000 years of mining.

Cliffside houses provide a spectacular view from their perch above the valley.