Stand at the center of this lakefront sundial, and you'll become part of the timepiece.

Crumbling remnants of what was once a booming town full of silver.

The scene of one of the darkest moments in American labor history has been left essentially untouched since the tragedy.

The largest tidal whirlpool in the Western Hemisphere.

Dig for your own trilobites in this goldmine of invertebrates.

Devilish landscape, once cinema home to giant alien bugs, now derelict roadside attraction.

Cliffs that hold fossils, including the earliest known reptile, dating back more than 300 million years.

See massive fossils that are over 200 million years old in northeastern Arizona.

This sacred cavern, 400 feet deep, is home to millions of bats and an ancient burial ground.

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

Small plaque marks the spot, but the true center is located about half a mile away on a private pig farm.

Mile-long cave system near Great Smoky Mountains.

More than five miles of underground passageways.

Curious cave that stays a cool 56 degrees all year long.

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

Once defending the city of New Orleans from invasion by sea, this fort faces an uncertain future.

An artificial crater field that trained astronauts for lunar exploration.

Iconic rock formations that allow for exploration during high or low tide.

Beautiful natural falls where Satan once ruined a pot of beans.

Ancient pueblos built into the base of a natural canyon wall.

The first declared National Monument in the United States.

Wee (and not so wee) footprints that insects and reptiles of the Paleozoic Era left behind.

The Yonaguni Monument, a natural formation or the Japanese Atlantis?

Limestone caves with spectacular showcase chambers.