A submerged archeological park still holds the wonders of a Roman Sodom.

A sulfuric hellscape in southern Italy.

Host to the annual Festival of the Snakes.

A cave in southern Italy adorned with thousand-year-old Biblical frescoes existed in obscurity until fairly recently.

Hideout of thieves, monks, and goat-footed gods.

A 2,000-year-old computer demonstrates remarkable engineering and astronomical precision.

A sprawling collection of books and manuscripts about ancient Greece that would have put their famed libraries to shame.

Large collection from the 3rd millennium BCE.

This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.

Ancient Greek oracle of the dead.

This field of half-buried stone crosses was the burial place of a sect of heretical gnostics.

A naturally beautiful, seemingly bottomless deep blue spring.

A miniature train-ride through Slovenia's largest cave system.

This cluster of little wooden huts once ground local farmers’ wheat into flour during the days of the Austro-Hungarian Empire.

Church holding the preserved body of a levitating saint.

This top-secret nuclear bunker hidden in a Bosnian mountain has been converted to a modern art exhibition space.

The secular incarnation of a religious meditation.

Seventeen oak trees sprout from the roof and walls of this tiny chapel without any visual evidence of roots.

The historic tower houses of this nearly abandoned Greek town have been compared to the spikes on an iguana's back.

This town named for the ancient God of War is home to fewer than a thousand people, but it's the birthplace of the Greek Revolution.

This remarkable cave was discovered after a shepherd watched bees enter and exit through a small fissure.

A cave network located at the southernmost tip of mainland Greece was once believed to be an entrance to Hades.

Week-long block party becomes a competition for the best decorated street.

Barcelona's historic 19th-century sewers, whose foundations were laid in medieval times, are open to the public for exploring.