The Bracken Bat Cave, located in southern Comal County, Texas, outside the city of San Antonio, is the summer home to the largest colony of bats in the world. Each year during summer, an estimated 20 million Mexican Free-tailed bats migrate from Mexico to Texas to give birth and raise their pups. While these bats take refuge all across the state, the largest congregation takes place at Bracken Cave.

Every night from March to October, these tiny creatures fly out of the 100-foot-wide crescent-shaped opening to feed. The bats’ exodus is slow at first as they linger at the cave’s lip, circling round and round inside before emerging. Over the next four hours, millions and millions of bats stream out of the cave in graceful swirls as they rise into the wide sky and dissipate in the distance. They spread out over a 60-mile radius from the cave at heights of up to 10,000 feet and over the next 8 to 12 hours feed on hundreds of tons of insects, such as moths and agricultural pests. Their emergence is so dense that it shows up on Doppler radar, according to TexasCoopPower.com. Those who have seen the bats’ nightly emergence have described the sound they generate as that of steadily falling rainstorm.

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