It's going to be a very noisy summer in some parts of the United States. After 17 years underground, a brood of periodical cicadas are ready to emerge into the world above in several east coast states.

Once they breathe in that first bout of fresh air and shed their underground coats, these harmless insects (of the genus Magicicada) have a mere four to six weeks to live, so there's no time to be shy.

As male cicadas begin singing out to silent females, using vibrating membranes on the sides of their abdomen, the resulting courtship can cause quite the racket.

With as many as 1.5 million cicadas expected per acre in Southwest Virginia, parts of North Carolina, and West Virginia, some residents are about to cop an earful.

"Communities and farms with large numbers of cicadas emerging at once may have a substantial noise issue," warns entomologist Eric Day at Virginia Tech.

"Hopefully, any annoyance at the disturbance is tempered by just how infrequent - and amazing - this event is."

It's the first time this brood - known as brood IX - has emerged since 2003. While some cicada species emerge annually, others mature underground for 13 or 17 years.

These are known as periodical cicadas, and their life cycle is amongst the greatest mysteries of nature we are yet to fully understand.

The unpredictable timing of it all might exist as a way to avoid predators, whose life cycle is much shorter, but currently that's just a hypothesis. We really have no idea why they spend such a long time maturing underground.

What we do know is that when soil-dwelling cicadas are mature enough to breed and lay eggs above ground, they emerge when the soil grows warmer, usually beginning in May.

This invasion is synchronised, and by June, their numbers have usually peaked, as has their noise.

Periodical cicadas are considered some of the loudest insects on Earth, and scientists say their alien-like wail can reach over 90 decibels, which is as loud as a lawnmower.

"Periodical cicadas are bugs of history," entomologist Gene Kritsky, who developed an app for tracking these insects, told West Virginia's Register Herald.

"They are generational events, and many people use the emergence to mark the passage of time, recall key events in their lives and just remember where they were and what they were doing the last time the cicadas came out."

Periodical cicadas have black bodies, red eyes and yellowish-orange wings. As weird as they look, however, they don't bite or sting, nor do they carry disease. Plus, they are usually too busy mating to feed much at all.

Still, cicadas do represent a threat to tree growers and orchard and vineyard managers, who need to plan ahead for the emergence of the swarms.

After mating, female cicadas lay hundreds of their small eggs in twigs and branches, which causes the branch or vine to split and wither. If enough parts of the tree are weighed down with eggs, it can actually kill them outright.

"Cicadas can occur in overwhelming numbers and growers in predicted areas of activity should be watchful," says entomologist Doug Pfeiffer from Virginia Tech.

As with most things in nature, however, there are two sides to this coin. Periodical cicadas are beneficial to ecosystems, too. Their emergence naturally turns over soil and prunes trees, increasing flowers and fruits in later years.

And they don't even live to see it. How selfless.