An underground city of ancient quarry tunnels hid thousands of Allied soldiers in WWI.

This former boomtown "island" is now submerged below Folsom Lake.

Concrete crosses scattered across the Arizona scrubland which were used to align spy satellites.

A cave dwelling that was started by the Celts, expanded by monks, and finished by knights.

Gladiators battle and chariots race daily in the ruins of a once-great Roman city.

This narrow path was cut right into a French mountainside to feed the country's early naval expansion.

A site where “the ravages of the war are more striking…than any other place in contemporary Spain.”

The famous boat that once ferried Bogart and Hepburn still floats in Key Largo after being abandoned for years.

This museum houses an imposing skeleton army comprised of creatures great and small.

The elaborate clockwork contraptions in this Scottish gallery create small worlds of whimsy and horror.

The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

Tens of thousands of rocks make up what is thought to be a giant star calendar.

The history of occult science is turned into a creepshow at this lurid Prague tourist attraction.

Once the tallest building in the world, this abandoned church is now a monument to the destruction of WWII.

Hidden deep in the Guatemalan jungle is a turquoise paradise of natural pools, caves, and waterfalls.

On the Oregon coast there is a natural hole that seems to be draining the sea.

This Russian locomotive museum was once devoted just to Lenin's funeral before expanding to encompass Moscow's entire train history.

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

The alpine falls where Sherlock Holmes apparently met his demise can actually be visited.

The world's first and only escalator for lazy bicycles and their daunted riders.

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

The only remaining collection of flayed figures made by French "madman" Honoré Fragonard.

Now home to nothing but a weather station and a landing strip, a group of enslaved people were marooned on this tiny island for 15 years.

This former Nazi castle was built as a school to train what would have been the future of the Third Reich.