The Eastern U.S. is about to see something that hasn't happened since the final episode of Friends aired on NBC: massive swarms of Brood X cicadas. Billions of the red-eyed, black-bodied insects are taking to the skies after 17 years underground, buzzing loudly to attract mates before they die. Not only is the spectacle of periodical cicadas unique in this country — it’s unique in the world, and biologists don’t really know why it happens. “It’s one of nature’s unsolved mysteries,” said Doug Yanega, senior scientist at UC Riverside’s Entomology Research Museum. “This is something that’s globally unique. You can’t see it anywhere else in the world. It’s a weird spectacle of nature.”