The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

A grove of colossal man-made flowers built to collect the sun's rays.

A 19th-century subterranean reservoir repurposed as a contemporary art cave.

Roman ruins top white travertine terraces formed by ancient hot springs.

This permanent collection of preserved bodies taken from Irish bogs is one of the leading authorities on such morbid mummies.

A railroad tunnel that, upon its completion, was considered one of the engineering marvels of the modern world.

"The most colorful grottoes of the world.

This ancient sinkhole-turned-healing-pool is a treasure trove of archeological discoveries and may even be the Fountain of Youth.

Giant pinnacles in the sun-scorched lands of Southern Utah have been nicknamed "white ghosts."

Created hundreds of years ago, the magical follies and landscapes at this park were almost lost to time before being restored.

A pagan tradition of draping trees with rags eerily lives on.

Extraordinary sculpture garden created by an eccentric English poet.

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

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

This large midwestern cavern features impressive rock formations and an underground waterfall.

These Great Lakes sea caves accessible by boat in the summer turn into temples of ice reachable by foot in the winter.

Acres of natural wonders left behind by a socialite-turned-gardener.

Nancy Thompson's Home from "A Nightmare on Elm Street."

The world's largest mountain carving could fit all of Mount Rushmore inside it many times over.

This 16th-century monastery has been turned into a surrealist architecture compound.

Fittingly, America's museum of architecture is itself a magnificently designed old building.

Julia Morgan's Moorish-Gothic labyrinth of fountains, trees, natural light and Oakland's notable dead.

An ancient Viking village in North America that predates Columbus by 500 years.

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.