Hear the sound as billions of cicadas emerge in US

Periodical cicadas are unique insects that have a 13-year or a 17-year life cycle, most of which they spend underground. They come to the surface in groups that scientists call broods.

Each brood with its descendants is named in a Roman number. This year, Brood XIX and Brood XIII are emerging together. The last time these particular broods emerged in the same year was two centuries ago in 1803.

The unusual co-emergence has led to high numbers of cicadas in many parts of the US.

This is what they sound like.