Known as Brood X, this massive swarm has been biding its time underground for the last 17 years. Brood X is part of a special group of cicadas called “periodical cicadas,” known for their singular life cycles. There are only 15 periodical broods in the world, all of them found exclusively in the eastern United States. Tens of thousands of years later, periodical cicadas are still emerging like clockwork every 13 or 17 years. Periodical cicadas are challenging to study, precisely because they have such long life cycles.