This incredible submerged cave network is the longest of its kind in the world and plays a vital role in the region.

The Hranicka Abyss is at least 1,325 feet deep, and likely goes down further. — -- A team of explorers believe they have discovered the deepest known underwater cavern the world in eastern Czech ...

I'm a severe claustrophobe. After getting trapped in a slide in my local McDonald's at five years old, I swore off all tight and dark spaces forever. So it's not entirely surprising that I couldn't ...

June 15, 2006 — -- "Nightline" joined forces with writer Tim Zimmerman and filmmaker Gordon Hiles, who documented an ambitious dive to one of the world's deepest, most dangerous underwater caves.

The origin story of Hells Canyon, North America's deepest river gorge, has long been unclear to scientists. But new research estimates it formed about 2.1 million years ago when a dramatic flood event ...