Trillions of enormous cicadas are starting to emerge from the ground for the first time in 17 years.

The huge bugs known as Brood X have reportedly been spotted in areas of the US after their long absence.

The insects, which stay underground for most of their lives before emerging to mate then die, form different broods which surface at different times.

Brood X last burrowed out of American soil in 2004, nearly 20 years ago.

Thousands of people claim to have seen them in their gardens in Washington DC, Detroit, Chicago and states including Ohio, Tennessee and Maryland, EarthSky reports.

Photos shared on social media show the bugs, which are roughly the size of a large adult thumb, crawling up trees and garden fences.

The noisy insects, which emerge when the soil temperature reaches 18C, are due to continue coming out across the US during the next few weeks.

They will lay eggs in tree branches, which after they hatch will burrow back underground and start the 17-year cycle again.

The creepy crawlies are notorious for constantly making loud screeching noises during their six weeks above ground.

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According to The Ringer, they can get to 110 decibels – which is roughly the same decibel level as a Metallica concert.

Although the creatures are harmless, they are said to remind people of the biblical plagues of locusts – although they are different species of insects.

National Geographic reported that Puritan settlers in Massachusetts in 1634 likened them to “swarms from the Old Testament”.

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Dr Michael Raupp, from the University of Maryland, told The Ringer: “Cicadas are not going to bite, they’re not going to sting.

“They’re not going to fly away with small children and dogs like the monkeys in The Wizard of Oz.

“These are harmless creatures.”

Angela Saenz, a graduate student at the University of Maryland, added: “They’ve been feeding on the tree sap, right on the roots underneath us, basically.

“And they’re all over the place.”

Social media users have already been freaking out after seeing them emerge.

One wrote: “Someone please save me from the trillions of cicadas that are about to wake up. They look very scary .”

Another said: “As someone who is terrified of insects this is basically more scary to me then any horror movie.”

But another wrote on Twitter: “They look & sound scary , but they're completely harmless.”

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