The ruins of a 19th century shelter run by the Independent Order of Odd Fellows.

This wooded grove of beautiful hand-built stone grottos is a hidden gem within a shuttered Franciscan monastery.

One man's unrequited love stands in a glass box forever.

A Mormon place of exile in Missouri.

The man who was president for only one day and spent most of it asleep.

Where the author wrote her beloved "Little House" books.

Like Stonehenge, but with boats and oodles more charm.

This historic cliffside mansion uses a natural cave for air conditioning.

A remarkably opulent mansion with a tragic history.

Giant granite rocks standing end-to-end like a train of circus elephants.

400 million-year old limestone caverns nestled in the Ozarks served as shelter and a hideout for cultures throughout North American history.

Erosion and time have created a naturally-formed water park.

If there's another drive-through golf-cart bar in a bat-filled cave on top of a mountain, we don't know about it.

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

European castle ruins in an American state park are actually the product of death and grief.

This abandoned prison castle has become an imposing gallery of graffiti art.

The ruins of the largest gladiatorial school in Rome are largely hidden under modern streets.

The ruins of an abbey believed to have been founded by Charlemagne in the 9th century.

Jesus' footprints and the first catacombs in the world.

Suffocating dogs was a common spectacle in the fumes of this noxious cave.

Before there were doorbells this bronze ear was installed for visitors to announce their arrival.

Long-abandoned art nouveau baths beneath Milan's Piazza Oberdan.

The secret medieval communication system will whisk your words above the noise of the crowd.

An ossuary built in 1210 to collect bones from the Ospedale Maggiore of Milan.