One of the rarest, and perhaps most irritating, natural phenomena has begun across the South: the emergence of the Great Southern Brood.

The brood — millions of plant-munching insects called cicadas — is finally crawling to the surface after 13 years underground. The cicadas will spend a few weeks mating and laying eggs. Then they'll all die.

In middle Georgia, the emergence is under way and the air is alive with an unending drone of mating calls, 24 hours a day.

"We've been humming here for a couple weeks now," said Carolyn Johnson of the Piedmont National Wildlife Refuge, just north of Macon, Ga.

Known as Brood 19, these cicadas crawl above ground every 13 years. They live in about a dozen states in the Southeast and Midwest and can be found as far north as Missouri and Illinois.

The brood is harmless, except to one's peace of mind. They make a constant electric hum that Kevin Kramer, of the Georgia Department of Natural Resources in central Georgia, likens to a muffled burglar alarm.

"We hear them every day," Kramer told OurAmazingPlanet. "You can't ignore it."