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

Two geological phenomena in one precarious locale: a towering stone window, and a deep blue void.

One of the deepest blue holes in the world.

A massive underwater sinkhole made famous by Jacques Cousteau.

An alleyway plaque is all that remains of the independent Texas' envoy that forgot to pay their rent for over a century.

This memorial commemorating fallen Swiss Guards of the French Revolution has been called the saddest stone in the world.

This historic covered bridge contains a series of macabre paintings that communicate one thing: everyone dies.

This amazing pedestrian suspension bridge in the Swiss Alps would give Indiana Jones nightmares.

A historic observatory precariously perched at the "Top of Europe."

Caves famous for the dragon-slaying saint who lived there.

A Gothic cathedral created from volcanic rocks.

This historic Idaho island is home to a strange timeshare museum, a homemade mini-golf course, and an outhouse named Aunt Fanny.

Discovered after draining a local pond, these ancient man-made caves are still a complete mystery.

An abandoned saucer-shaped monument in the Bulgarian mountains.

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

A small flooded ghost town with a few scattered structures remaining.