Right now, across much of the midwestern and eastern US, trillions of cicadas are crawling out from the soil. And this year is extra special, because two broods are erupting from the ground at once. The first brood hasn’t been seen for 13 years, the other for 17 years and the last time they emerged together Thomas Jefferson was president. Ian Sample speaks to entomologist Dr Gene Kritsky to find out what’s going on, why periodical cicadas emerge in cycles of prime numbers and how they keep time underground. Help support our independent journalism at theguardian.com/sciencepod