Now they have arrived in some states and people are freaking out about the noise.
Trillions of cicadas are emerging in states including Illinois and Georgia and they are extremely loud.
In one South Carolina county, residents are calling the sheriff's office or flagging down deputies asking why they can hear sirens or a loud roar.
The Newberry County Sheriff's Office had to put out a Facebook message this week letting people know that the sound is just the male cicada singing to attract a mate after more than a decade of being dormant.
Two different broods are appearing for the first time in 200 years. One appears every 17 years, while the other appears every 13 years.
Thankfully, they're concentrated primarily in the Midwest and Southeast parts of the country.
If you travel to those parts of the country and hear a loud hum, do not call police.
Apparently, the cicadas are as loud as a jet engine and scientists even wear earmuffs when studying them.