"CAVES
Airs Sunday, April 22, at 9 p.m. ET/PT
Caves are Earth's final frontier, and this episode will go where few have been before. Caves are one of the only habitats not directly driven by sunlight, but this doesn't mean there's no wildlife living in their confines. Descend into darkness to witness the unseen behaviors of bizarre creatures like cave angel fish that attach themselves to walls and swiftlets that build nests from saliva." (from http://www.planet-earth.com)
From a few published articles and an interview in the April DISCOVER magazine, the photography is promised to be spectacular.