A subterranean Holocaust refuge within one of the world's longest cave systems.

Sail through this subterranean show cave that is home to a legend that would go on to name nearly everything around it.

Reaching the "Church in the Sky" requires scaling the cliff face of a sandstone pinnacle with a 650-foot drop.

Ethiopia's "smoking mountain" is in one of the hottest regions on the planet and is home to two lava lakes.

An ancient Ethiopian cave church is endangered by the simple building of a road.

This neon lit Georgian cave claims to hold the boulder Prometheus was chained to.

The true depths of these stunning Moroccan caves are unknown.

The massive cave church of Cairo's Zabbaleen community is one of the largest Christian churches in the Middle East.

The ceiling of Prohodna Cave in northern Bulgaria affords a peek at the sky via two deity-sized-eye-shaped holes.

The underground sanctuary of a mysterious ancient cult.

The largest cave chamber in Britain—big enough to fit a whole cathedral inside.

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

Nearly 40 miles and 1,000 years of limestone mines under Denmark.

A scenic canyon with thick forests and historic ruins.

An army of sleeping knights sculpted by a self-taught local artist beautifully depict an old Czech legend.

Who needs fancy modern water slides when this giant North Carolina rock works just as well?

For centuries no one knew there were hundreds of bones and precious artwork hidden beneath this Swiss church.

When the public was consulted about a new piece of street art, they naturally asked for an enormous metal dragon.

The ruins of a former sanctuary for elite Aztec warriors.

The remains of an Aztec school and various intriguing artifacts lie below a modern cultural center.

The ruins of an ancient temple to the Aztec wind god discovered in the middle of a metro station.

Italian cave with exceptional acoustics and an unsavory history.

Built on a medieval synagogue, this church has a burial crypt, tunnel system, and Jewish ritual bath hiding deep underneath.

One of the oldest churches in Romania is believed to have been built on the site of a pagan temple.