The gaping mouth of Bracken Cave, created when limestone collapsed into a sinkhole in this rocky patch of Hill Country millennia ago, reveals only darkness to the human eye. Yet the room inside is filled with the pulsing life of tiny, furry, pregnant Mexican free-tailed bats in April—a massive maternity ward that will, by summer, swell to 20 million after each mother gives birth.
This cave, a few miles north of San Antonio, is said to hold the world’s greatest concentration of mammals in such a compact space.
