Cicadas are awakening 4 years early. It’s not a sign of the apocalypse but a scientific mystery.

In 2004, billions upon billions of shrimp-size insects took to the skies in the mid-Atlantic United States, covering entire houses, blaring extremely loud noises, and littering forests with their exoskeletons. When the swarm was finally over, the insects — cicadas — laid their eggs in the ground. Residents were told these creatures wouldn’t emerge again for another 17 years.

The cicadas had other plans.

Reports in Baltimore and the Washington, DC, metro area indicate that a small group of the cicadas — a group called Brood X — are emerging from the ground to molt, mate, and make noise. They weren’t supposed to return until 2021.

No, this isn’t a sign of the impending apocalypse. (We think. We hope...) But entomologists are confused as to why portions of the brood, which have only been underground for 13 years of their 17-year cycle, are waking up and swarming. It’s just one more mystery of these amazing creatures, which have an uncanny ability to keep time.

To be clear, the bugs waking up now appear to be just a small fraction of the billions of cicadas that make up the enormous Brood X. “So far, the current batch of early-arriving cicadas are sparse in numbers and are quickly falling victim to feasting birds,” the Washington Post points out. We know they’re from Brood X because of where they’re popping up. Different cicada broods occupy different parts of the country.

And so there’s no need to panic, according to the cicada enthusiast website Cicadamania: “Less that one percent of a Brood [wakes up early]. If you had 10,000 cicadas in your yard back in 2004, you can expect a less-frightening or more manageable dozens or hundreds.”

Cicada broods lay claim to much of the eastern United States, stretching from New England to Oklahoma. You can see all the United States broods on this US Forest Service map below.

Each year a different, massive 17-year-old crew awakes — likely an evolutionary strategy to survive. Cicadas aren’t particularly nimble and make for easy meals for birds. But if billions of cicadas appear all at once, predators (like birds or small mammals) can’t make a meaningful dent in their numbers.

For 17 years, the cicadas do very little. They hang out in the ground, sucking sugar out of tree roots. Then after this absurdly long hibernation, they emerge in the spring. They molt out of their juvenile bodies, sprout wings, make a ton of noise, have sex, and die within a few weeks (after making a few meals out of sucking on tree fluids). Their orphan progeny will then return to the ground and will live the next 17 years in silence.

(Note: There are some broods of cicadas that live just 13 years in the Southeast. Some researchers suspect that the 17-year broods and the 13-year ones are slightly different species.)

If you lived through a cicada brood awakening, you know it’s like listening to a thousand leaf blowers turned on all day and night.

The reason: It’s how they attract mates. The females make a clicking noise with a flip of their wings. The males — which make a buzzing noise reminiscent of a lawnmower — find these clicks to be very sexy. They will immediately divert their attention if they hear a female click nearby.

Here, esteemed nature documentarian Sir David Attenborough demonstrates by seducing a cicada himself. “I can imitate the female’s wing flip with a snap of my fingers,” Attenborough says in his unmistakably husky voice in this clip from a BBC program below.

Portions of a brood waking up early are not unheard of, but experts are still unsure of why it happens, and why it is happening this year with Brood X. Something has interfered with the insects’ ability to keep track of time. One theory: climate change. As the Baltimore Sun explains, “longer growing seasons linked to climate change may have shortened the life cycle of many 17-year cicadas, and could end up creating new cycles of timekeeping broods.”

(It could also just be that cicadas are more carefully monitored now than they have been in the past. Scientists use crowdsourcing websites to help monitor broods. It could be that portions of broods almost always wake up early but we haven’t noticed.)

We also don’t know how cicadas count to 17 years in the first place. And for that matter, no one knows why cicadas are fond of prime numbers like 13 and 17 either. “Some researchers have proposed that the cicadas have evolved life cycles around prime numbers of years because that arrangement limits the chances that predators will synchronize with the cicadas,” Nature writes. (That is, the birds that eat cicadas are unlikely to adapt to a 17-year cycle to prepare for the feast.)

But that’s just a guess.

Brood X may be the surprise guest of 2017, but this year is also Brood VI’s time to shine.

Here’s a closer look. The brown dots are where this year’s Brood VI cicadas are supposed to emerge.

The Baltimore Sun reports that scientists will continue to track the weird wake-up of parts of Brood X. And there could be more on the way. Cicadas often don’t wake up until the soil reaches 64 degrees. With a hot, hot week underway, more could be working their way to the surface as I type.