Over a hundred years ago, in the sparsely populated forests of Siberia near the Tunguska River, a massive explosion shook the earth. It flattened 83 million trees over 600 miles of forest. It destroyed livestock herds, shattered windows, and knocked a man right out of his chair—from 40 miles away.The Tunguska event, as it's come to be called, was the result of either a comet or meteorite (probably the latter) exploding from air compression force before it hit the ground. It's classified as an impact event, despite the fact that the space object broke up in the sky rather than actually hitting the ground. Still, the explosion was powerful enough to measure a 5.0 on the Richter scale. And for several days afterward, aurora-like "night glows" were bright enough that you could read a newspaper in the middle of the night.As this explainer from Atlas Obscura shows us, scientific investigation of the Tunguska event didn't really begin until the 1920s, when mineralogist Leonid Kulik made the trek to the explosion site. The remote location of the event and the lack of any recorded fatalities just didn't get anyone fired up enough to figure out what had happened.Nowadays, though, Tunguska is recognized as remarkable by scientists, because it remains the only major impact event with existing first-hand accounts. Don Yeomans of NASA's Jet Propulsion Laboratory says, "If you want to start a conversation with anyone in the asteroid business, all you have to say is Tunguska." He estimates that a similarly-sized impact event only occurs every 300 years, so don't let it keep you up at night.