Right now, something magical is underway across vast stretches of the eastern US: The soil is beginning to erupt with trillions of periodical cicadas that have been growing underground for either 13 or 17 years, waiting for this exact moment to crawl out of the Earth together.

This particular burst of life is incredibly rare. The two groups that are emerging, known as Brood XIX and Brood XIII, appear together just once every 221 years. Brood XIX are 13-year cicadas, and are showing up in southern Illinois, Missouri, and parts of the southeast, whereas Brood XIII are 17-year cicadas that live in and around northern Illinois.

That makes this event more rare than a total solar eclipse in North America. And depending on who you ask, it’s just as spectacular.

Cicadas are, in fact, spectacular bugs. Although they have tiny insect brains, they can count and sense temperature; males have built-in drums that make noises so loud they can damage human hearing; and in just a few short weeks they transform entire forest ecosystems.

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So yes, while they may be obnoxiously loud and, to many people, extremely icky, periodical cicadas are an ecological wonder — and one that we arguably take for granted. It’s worth getting to know these bizarre bugs a bit better, starting with these eight incredible facts:

1. Cicadas count years, likely using the flow of tree sap.

Eastern North America is the only place in the world where you find periodical cicadas — groups of cicadas that emerge from the ground every 13 or 17 years, depending on the brood, or group. Slightly different kinds of cicadas, known as annual cicadas, appear every summer and have a global distribution.

For all of those years, periodical cicadas are several inches or more under the soil, sucking sap from the roots of plants using a straw-like mouth. Then, at the same time and in the right year, they all surface together.

And it’s important that they do: Bursting from the ground all at once overwhelms their predators, which include birds and squirrels (and dogs). There are simply too many cicadas to eat, so plenty of them survive and can seed the next generation.

But how do they pull off such a stunt of time-keeping? The key may be in the root sap, known as xylem fluid, that cicadas drink while they’re underground.

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Throughout the year, as trees grow and shed leaves through the seasons, the flow and makeup of that fluid changes. It functions like an internal clock. Cicadas can likely detect those changes to keep track of the years, according to Martha Weiss, a cicada researcher at Georgetown University.

The bugs then use their ability to sense soil temperature (among other things) to know when in the year they should emerge. Typically, it’s when soil reaches roughly 64 degrees Fahrenheit.

2. Sometimes their clocks screw up (courtesy of climate change?).

The numbers 13 and 17 are important for these bugs, though it’s not totally clear why. Scientists suspect that surfacing at this unusual cadence makes it hard for their predators — which include basically everything in the forest — to anticipate their emergence. Birds and other animals that eat them typically won’t experience more than one eruption of a particular brood in their lifetimes.

“Their ability to track [the emergence] is almost impossible,” said John Lill, a biologist at George Washington University who studies cicadas.

On occasion, however, periodical cicadas seem to get their math wrong. Some come out of the ground at nine years, whereas others will emerge at 21 years, Weiss said. It likely has to do with their size: If the young cicadas are growing faster than normal, they might come up four years early, whereas if they’re developing more slowly, they may wait another four years to emerge.

The reason they choose four-year intervals is a mystery. “There’s something magical about four years and four-year intervals, and we don’t know what it is,” Weiss says.

Remarkably, because climate change is extending the growing period for trees in temperate regions, it might make cicadas develop faster, potentially turning some 17-year cicadas into 13-year cicadas, according to Weiss.

3. The current cicada explosion gets a lot of attention, but there are actually two of them. The second one goes largely undetected.

The big cicada emergence is starting now in warmer parts of the southeastern US. That’s when billions of bugs crawl out of the Earth, sometimes from little mud turrets that they build. Once they emerge, the cicadas scale plants, molt into winged adults, mate, lay eggs, and die.

But that’s not the end of it. Several weeks later, the babies hatch from slits on tree branches — filling the forest once again with bugs. “That’s a whole second pulse of cicadas,” Weiss said. This second pulse goes mostly unnoticed because hatchlings are mere whispers of the buzzing hordes they will one day become.

These newborns are white and just a couple of millimeters long. After hatching, they scurry along twigs in the canopy and they jump off, “floating like snowflakes down to the ground,” Weiss said. Then they’ll burrow into the Earth, assuming they don’t get eaten first by ants and other predators waiting on the forest floor.

Few scientists have studied this second emergence, said Weiss. She’s traveling to Illinois later this month with Lill to figure out how these baby bursts might impact a whole range of services that ants provide, from spreading seeds to protecting aphids (and sucking sweet honeydew from their rear ends).

4. Cicadas are incredibly bad at not being eaten.

During a cicada emergence, the forest is chaos — everything from large and small birds to lizards and squirrels shovels the bugs into their mouths. For animals, these broods signify the coming of an incredible, singular feast. This is an eating holiday in the animal world that we humans can’t rival.

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While great for hungry animals, this isn’t good for cicadas, which couldn’t be easier targets. They’ve got bright red eyes, which pop against the forest palette. They’re also extremely, irritatingly noisy, making them easy to locate. Plus, they have no defense — no bite, no sting, no venom.

Adding to their woes: They’re not even good fliers, Lill said.

What’s interesting, he said, is that annual cicadas are much better at defending themselves against predators than the periodical ones. They’re larger, well camouflaged, and better fliers. And that points to a difference in strategy: Periodical cicadas rely on defense in numbers, whereas the annual ones have evolved more bug-level predator avoidance systems.

5. They rewire entire ecosystems.

Although periodical cicadas only appear for a few weeks, their blip above ground can rewire entire forest food webs, with knock-on effects that last for years.

Consider birds. They have so much to eat during cicada eruptions that they may actually produce more chicks in the months that follow. A study from 2005 linked cicada emergences to a population bump in a number of species including red-headed woodpeckers and common grackles.

“Following emergences, you do tend to get an increase in a lot of the apparent avian predator populations,” Walt Koenig, the lead author of the 2005 study and an ornithologist at Cornell University, told me in 2021.

In fact, the birds are eating so many cicadas that they’re consuming much less of everything else — including caterpillars. That means caterpillars get a rare reprieve from the constant threat of attack, at least from birds.

Scientists, including Zoe Getman-Pickering, an ecologist at University of Massachusetts Amherst, actually measured this caterpillar effect during the Brood X emergence in 2021. Here’s how I described their experiment in a recent story.

In the years surrounding Brood X, Getman-Pickering and her collaborators filled forests in Maryland with fake caterpillars made of clay. They then measured how many of them had signs of bird strikes — beak marks indicating that birds tried to eat them. In May, when Brood X was emerging, the portion of caterpillars with strike marks fell dramatically, from about 30 percent in a typical year to below 10 percent during the emergence, according to her study, published in 2023. She also looked at real caterpillars. Remarkably, the number of them roughly doubled in the forests she studied during the emergence, relative to the two following years.

The large number of caterpillars also impacts trees, because these larvae eat leaves. Getman-Pickering’s research showed that oak trees experienced “a spike in cumulative leaf damage” after Brood X debuted.

Cicada booms can also shape forests in more subtle ways.

This spring, Weiss and Lill will study how eruptions influence ants and their behavior in the forest. Ants help wildflowers disperse their seeds. Many of those seeds have little growths on them called elaiosomes. Elaiosomes are rich in fats, and they look and smell a bit like dead insects, making them appetizing to ants. The ants bring them to their colonies, remove the growth and eat it (or feed it to their larvae), and then discard the seed outside their nests, where they sprout.

The scientists suspect that if ants have an endless buffet of cicadas they might be less likely to disperse plant seeds.

6. A weird fungus can turn cicadas into zombies.

Most animals, like birds, just eat cicadas because they’re there — they’re everywhere. But there is one enemy that specializes in periodical cicadas: Massospora cicadina, a fungus.

It’s a bizarre species with a very smart strategy. The spores, which lay dormant in the soil for years underground, infect the cicadas after they emerge. The fungus then grows within their abdomens and eventually bursts out of their rear-end, causing their sexual organs to fall off.

But wait, there’s more: Males infected with the fungus not only call to females, as is expected, but also start mimicking female sounds (made with a flip of their wings) to draw in males. This helps the fungus spread. When two bugs try to mate, they spread the infection to one another. It’s essentially a sexually transmitted infection that benefits by juicing this insect orgy.

7. Different species have different songs.

The hum of periodical cicadas in the summer is loud and perhaps obnoxious but it is, in fact, a serenade. It’s produced by males who are calling to attract females.

The insects make those sounds by vibrating membranes inside drum-like organs called tymbals, which are then amplified by their mostly hollow abdomens. Interested females will then respond with a far more subtle wing flip that makes a clicking noise.

The eastern US is home to seven different species of periodical cicadas. And remarkably, the sound of each is slightly different. All seven species are emerging this year, and somehow these insects can tell each sound apart. Not only that, but males have different calls for different phases of their courtship.

8. They are very much edible and tasty.*

*…According to one person I talked to.

But really — some people do eat them and say they taste good. The key is grabbing the insects right after they’ve molted, Weiss said, before their bodies harden and they become fully formed adults.

From there, you can choose whether to go the sweet or savory route.

“The chocolate-covered cicadas are kind of the entry level insect,” said Weiss. Dry roast the bugs and dip them in chocolate, she said. “They taste like a chocolate-covered pecan.”

Alternatively, you can pluck up the nymphs before they’ve molted, toss them briefly in a pot of boiling water, marinate them in teriyaki sauce, and then pop them on the grill.

“They’re crispy and crunchy,” Weiss said. “They don’t have as much meat as shrimp but they’re not that different.”

“We’ve rebranded them as tree shrimp,” Weiss said. “There’s a slight hypocrisy in that people are happy to eat shrimp and lobsters but are grossed out by cicadas.”

A version of the story appeared in Today, Explained, Vox’s flagship daily newsletter. Sign up here for future editions.