More than five miles of underground passageways.

Labor provided by Great Depression-era programs restored these 25,000 acres in Tennessee.

Mile-long cave system near Great Smoky Mountains.

Enormous lake at the bottom of a unique cave system.

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

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

A stunning silver lining to global warming's gloomy cloud.

Collapsing castle on the Hudson river was once the storage site of military surplus business.

A tunnel dug through the side of a mountain by hand provides convenient yet death-defying access to a small village in China.

Beijing's World Park offers the excitement of international sites without the travel.

Two stunning 17th-century libraries and a cabinet of curiosities.

Monasteries on rock pillars, once accessible only by frayed ropes.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

This hunter-gatherer architecture is believed to be the oldest religious complex known.

The crypt of the imposing Stephensdom holds royal intestines and thousands of skeletons.

A Belgian self-built tower inspired by the Bible and ancient civilizations.

At the foot of a waterfall lies a peaceful swimming lagoon and evidence of over 60,000 years of human habitation.

Abandoned Nazi train station turned underground astroparticle laboratory.

Abandoned castle was once a grand example of Russian privilege, built exclusively to win a frivolous bet between noblemen.

Gigantic unfinished stupa and the world's 2nd largest bell.

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

Mystic Incan rock piles three miles above sea level.

Mysterious lost empire of the Silk Road.

This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.