On July 4, 1999, a rare and terrifying storm swept through the Boundary Waters Canoe Area Wilderness in northern Minnesota. What began like a standard-issue thunderstorm soon turned strange and fierce, generating green clouds and strong winds reminiscent of a tornado. In fact, the storm was a cousin of the tornado: a derecho (pronounced "de-RAY-cho"), a type of storm so infrequent and fast-moving that only in recent years have meteorologists begun to understand how to recognize and forecast it. This feature tells the story of this unusual event and the efforts of scientists to understand it.