A collection of huge Pre-Colombian structures in Alabama.

The marker offers a rare chance to be in four U.S. states at once, though there's some controversy over its geographic accuracy.

A small metropolis of Pueblo cave dwellings have been carved right into the hillside of this national monument.

Visit this playground of waterfalls, caves and sinkholes.

This prison-turned-museum is a remnant of the Old West's grizzly, violent past.

The perfectly named cave drop is the deepest in the continental United States, almost matching the height of Seattle’s Space Needle.

An Industrial Revolution-era public work that purified water using nothing but sand.

In the woods of New Jersey lie the ruins of a munitions village abandoned after World War I and the remnants of a 1920s wooden racetrack.

Hidden in the Chihuahuan Desert resides an amazing collection of petroglyphs.

This unusual 7th-century Frankish seal offers more questions than answers about its past.

This once-popular otherworldly landscape was formed by one of the largest volcanic eruptions in history.

This magnificent, quirky home sits atop a literal gold mine in the middle of the Nevada desert.

According to local lore, this narrow passageway through two large rocks was accidentally created by the devil himself.

This head-spinningly bizarre Depression-era art cave is wall-to-wall quartz and Jesus.

Eons old volcanic activity created one of the most stunning rock formations in the United States.

A legal loophole makes it possible to get away with murder within this 50-square-mile section of Yellowstone.

Explore the dark underground corridors of this graffiti-covered former military fortification.

Legend has it the mill owner's daughter, Mary Musgrove, helped a Patriot soldier hide behind these falls.

These azure waters were thought to lead to the Mayan Underworld.

An Aztec snake god has taken the material form of an apartment complex.

"Dark Sky Park" is heaven on Earth for astronomers, one of the rare U.S. locations with no artificial light pollution.

This roadside stop between Goblin Valley is the perfect location to view some of the region's natural wonders.

This set of three large, volcanic hot springs is a favorite haunt for intrepid divers.

The unusual patterns in this rock formation result from two distinct modes of weathering.