Bracken Cave, outside San Antonio, Texas, is a protected habitat and the summer home of an estimated 20 million Mexican free-tailed bats, the largest known concentration of mammals on Earth. From March to October, bats return from Mexico, and the cave becomes a densely packed nursery where females give birth. Their enormous evening emergence flights and their consumption of insect pests connect the cave directly to surrounding farmland, saving south-central Texas cotton farmers an estimated $1.2 million a year. Protected by Bat Conservation International and other partners, the cave is carefully managed to preserve both its habitat and one of North America’s most significant wildlife gatherings.