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Closeup image of cicada.

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Two broods of cicadas will surface later this spring in portions of the Midwest and Southeast after years of hibernation. As per the University of Connecticut, there could be a trillion cicadas in the world.

Periodically, the two broods emerge above earth just once every 13 or 17 years. Every 17 years, the Northern Illinois Brood, also known as Brood XIII, resurfaces. This spring, Brood XIX—the Great Southern Brood—will join, occurring every 13 years.

Maps show where in the U.S. to see cicadas

Since 1803, this year is the first that the two broods have appeared in the same year. The New York Times reports that it won’t occur again for 221 years. The University of Connecticut states that although there is some overlap, the two broods are not very great.

Cicadas are so noisy in South Carolina that residents are calling the police. People are mistaking their loud mating calls for sirens or a roar.

The Newberry County Sheriff’s Office clarified that the noise is just male cicadas singing to attract mates after being dormant for over a decade.

Despite the annoyance, the cicadas pose no danger to humans or pets, and their emergence is a natural phenomenon that occurs on 13 or 17-year cycles.

Scientists studying these red-eyed insects often wear earmuffs to protect their hearing from the jet engine-like noise produced by the cicadas.