The subspecies of cicadas known as Brood X has been underground for the last 17 years, tapped into tree roots, feeding and fattening up.

NPR: Roar of the Cicada

Over the past month, the periodical cicadas of Brood XIII, native to the area, have crawled up roots, through the earth's surface and onto tree trunks everywhere, to moult and spread wings.

ECONOMIST: Cicadas in Illinois