Photography by Carsten Peter Headroom shrinks in the middle of Hang En as cavers pass beneath a ceiling scalloped by eons of floodwater rushing past. The river shortly reemerges onto the surface, then burrows into Hang Son Doong after a few miles.
Hang Son Doong skyhole by John Spies. The Son Doong Cave in Vietnam is the biggest cave in the world. It's over miles long, has a jungle and river, and could fit a 40 story skyscraper within its walls. It was discovered by modern man 5 years ago.