For the first time in 221 years, this spring will see billions, if not trillions, of cicadas take to the skies in a rare synchronized event that will transform our ecosystems for years to come.

In forests across the United States, two groups, or “broods,” of these noisy insects will crawl out from their underground dwellings to sprout wings, mate, lay eggs, and eventually die. In the Midwest, there’s Brood XIX, which pops up every 13 years, and Brood XIII, which emerges every 17 years and is concentrated in the Southeast.

The mass eruption, scientists believe, is strategic, but many mysteries about cicadas remain: Why do their alarm clocks use prime numbers? For that matter, how do they keep time? We’ll explain everything we know about this spectacular double brood event here. Follow along.