The largest drain hole in the world.

This unique structure in suburban Austin is made up entirely of other people's castaways, and constantly changing.

Abandoned bridge hidden deep in the San Gabriel Mountains.

Giant, natural amphitheaters made of delicate geological formations called "hoodoos."

Explore 10 acres of environmental sculptures.

A stunning, two-tiered waterfall made famous by poets, artists and the legend of a ghost dog.

Aqueducts have dramatically changed this old lake, now home to tufa towers and its very own species of tiny brine shrimp.

Odd outsider art installations surround a Gold Rush-era ghost town.

The first declared National Monument in the United States.

A theme park that suffered massive flooding during Katrina was abandoned, and now proves irresistible to trespassing explorers.

Towering basalt columns, a creation of fire and ice.

Eighteen miniature death-scene dioramas.

One man has carved a number of natural New Mexico caves into psychedelic sandstone temples.

An emerald-green grotto just a short trip from Austin.

For two weeks every June, fireflies in the Smoky Mountains perform an astonishing light show.

This beautiful building in the heart of Manhattan's Financial District was empty for decades.

Dramatic river bend surrounds a natural red-rocked pedestal.

Most-visited and most-photographed slot canyon in the American Southwest.

World's first "penitentiary," meant to be humane, drove men insane.

Rippling sand dunes frozen in the Arizona rock.

An artsy hidden staircase leads to breathtaking views of San Francisco.

Non-profit time travel mart sells goods you would need for a trip through the fourth dimension.

A terrific purveyor of natural history objects and curios.

During prohibition, corrupt city officials ran drinking dens under the streets of Downtown Los Angeles.