In the eastern United States, Magicicada cicadas emerge from the ground every 17 years. This is known as "Brood X". Billions of them invade gardens, cars and forests. As adults, they return to the surface to reproduce, lay eggs under tree bark and then die. Research suggests that this long period may be a way of avoiding predators. The last major invasion was in 2021. We'll have to wait until 2038 to hope to see them again.