The longest continuous lava tube in the continental U.S. offers 2.5 miles of dark explorations for the adventurous.

Limestone caves with spectacular showcase chambers.

These sub-farmland caves are home to a majestic stalactite known as the "Crystal King."

This massive underground salt mine now holds a sci-fi theme park in its depths.

This massive subterranean fissure is known as the "underground Grand Canyon."

The true depths of these stunning Moroccan caves are unknown.

The wartime boat shelter now doubles as an underground concert hall where musicians play on rafts along the waterway.

The remains of a community that was forced to move for a dam that never materialized still rot in this Pennsylvania park.

Hundreds of hopeful gold miners came here to get rich, but only two years later the small town was deserted.

This tiny New Jersey village has lived many lives but seems determined to remain a ghost town.

Mining town from the late 1890s, once home to Calamity Jane.

Hidden in the hills of Western Pennsylvania lies one of the best-kept ghost towns in America that you can legally visit.

This former mining hub holds a surprising abundance of undisturbed relics from the Old West.

This deserted Gold Rush town is home to a Masonic lodge that, strangely enough, still holds meetings.

This underfoot portrait is the work of a disgruntled painter inspired by a tragic poem.

The burial grounds of a secret society includes fences topped with fake Tesla balls.

This unassuming building houses one of the world's most extensive collections of cars created by automotive legend Carroll Shelby.

This now-abandoned outcrop is an important Native American spiritual site, overtaken by European settlers and mining pollution.

The ruins of this medieval castle in Northern Ireland have inspired artists from C.S. Lewis to Led Zeppelin.

The atmospheric road was never meant to be so creepy.

Along the most northwestern point in Africa is an ancient cave surrounded by legends.

Stepping away from the glitz of mobsters and casinos to learn about the other history of Vegas.

Thousands of artifacts relating to human sexuality, including the world's largest sex bike.

Formerly the world's largest, this Willy Wonka-esque contraption runs 24/7.