The true depths of these stunning Moroccan caves are unknown.

The world is full of swimming holes, but few compare to Samoa’s idyllic To Sua ocean trench.

A clear blue swimming hole with hidden caves, still unexplored.

What was once a presidential estate is now an exotic menagerie and home to the Budweiser Clydesdales.

The remains of this deep, dark mine have left behind a haunting rocky warren of tunnels.

A tiny microvalley etched into the side of Knocknarea Mountain hides another world.

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

Visit this playground of waterfalls, caves and sinkholes.

This modern castle sat empty for decades before a fire inspired its builders to complete its luxury halls.

The world's longest known cave system.

Where hills of dirt conceal the burial ground of one of the most powerful ancient Native American societies.

Once an ambitious nature spa, these forest ruins now provide a serene monument to failure.

The longest navigable underground river in the United States is home to countless blind and albino animals.

Ornate limestone building now filled with geologic specimens, and more.

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

Ruined pieces of stone and a swimming pool are all that remain of an amusement park swept away by a flood.

This former tuberculosis research hospital endeavored to cure the dreaded disease with fresh air and positive attitudes.

This abandoned 19th century orphanage started as a boarding school for children of Civil War veterans.

A historic house in St. Louis was plagued by a series of suicides committed by family members involved in a successful brewery.

"The House That Love Built" is shying away from "The House That Ghosts Love."

These historic defenses ensured the city within would survive hundreds of years of occupations, sieges, and invasions.

If you’ve ever seen a picture of Machu Picchu, then you’ve seen a picture of Huayna Picchu—you just didn’t realize it.

This Japanese-inspired labyrinth of hot springs is hidden in a Chilean forest.

The world's largest man-made underground caverns offer unparalleled underwater wonders.