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

The largest of London's subterranean rivers and once a mad, bad center of London life.

This natural French spring has been turned into a magical grotto by Roman and medieval constructions.

An abandoned lighthouse awaits its destiny: being buried under shifting sand.

A "simple-hearted artist" turned an isolated Icelandic valley into his own art museum.

These molds for fiberglass statues have formed an eerie, accidental sculpture park.

This tiny Bulgarian town offers a stunning location in the mountains and a storied history.

The world's oldest rose is so tough it survived being bombed in World War II.

The Pope’s secret escape route.

A house filled to the brim with oddities and curiosities.

A 100-year-old purveyor of curiosities, curios, and kitsch featuring mummies, shrunken heads, and a Fiji Mermaid.

A walk through the historic city center, under the streets of Seattle.

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

Like something out of a pulp adventure, this abandoned plane wreck is slowly being retaken by the Caribbean jungle.

While only two of the ancient buildings at this site are actually churches this was one of the earliest Irish pilgrimage sites ever.

Despite being pillaged and destroyed a number over times down the centuries, this historic church still sort of stands.

This Nazi ruin was the site where Hitler was almost blown up by a suitcase bomb.

The culture that built these Tanzanian ruins displayed advanced knowledge of irrigation technology before mysteriously disappearing.

The Uyghur settlement in the middle of a lake continues to raise questions.

The remains of this ancient fortress atop a manmade hill look as though they were built into a volcano.

This French castle is said to have once held the Holy Grail but is now a gorgeous mountaintop ruin.

A set of ancient drawings in the desert of Peru that are so massive, many of them can only be fully seen from the sky.

A piece of John the Baptist in the American heartland.

The wooden octagon in which Huckleberry Finn and A Connecticut Yankee were born can still be visited in upstate New York.