Photo of the Day 29

Bats in Cueva de Villa Luz, Mexico, 2001

Photograph by Stephen Alvarez

Bats emerge from the inky depths of Cueva de Villa Luz in southern Mexico. Hydrogen sulfide, which is poisonous to humans, permeates the cave's walls, streams, and air, sustaining a rich variety of bizarre organisms. In addition to bats, spiders, mites, amblypygids, and small, hemoglobin-rich fish thrive in the sulfur-saturated environment.

(Photo shot on assignment for, but not published in, "Deadly Haven," May 2001, National Geographic magazine)