Bracken Cave

Bracken Cave is the summer home to the largest colony of bats in the world. An estimated 20 million Mexican Free-tailed Bats roost in the cave from March to October making it the largest known concentration of mammals. The cave is located in southern Comal County, Texas, outside the city of San Antonio. The 100-foot-wide crescent shaped opening to the cave lies at the bottom of a sinkhole, formed when the roof of the cave collapsed.

• http://en.wikipedia.org/wiki/Bracken Cave