The first twister touched down around 1 p.m. Others quickly followed on that breezy, rainy afternoon in March. Low-slung and packing winds as high as 300 miles (480 kilometers) an hour as they rampaged across the heartland, they looked to one survivor like clouds eating the ground.

By the time the storms were finished three and a half hours later, 689 people lay dead or dying along a 219-mile (352 kilometers) trail of destruction across Missouri, Illinois, and Indiana. The Tri-State Tornado Outbreak of 1925 was especially treacherous because many of the twisters came in so low that they couldn’t be seen at a distance.

An average of 800 tornadoes are reported in the United States every year, every one a potential killer. Many occur in Tornado Alley, the central Plains from Texas to Nebraska. But they can happen anytime, anyplace—even on the side of a mountain. An outbreak in May 1999 that swept from Texas to the northern Plains was one of the costliest in U.S. history, with an estimated $1 billion dollars in damages. On May 3, in the Oklahoma City suburbs alone, 36 people were killed and more than 300 were injured.

Flying debris and winds potentially more violent than a hurricane create most of the havoc. At their worst, tornadoes can rip apart ordinary houses, toss cars into the air, and turn small objects into bullets. Waterspouts—weaker marine cousins that form over warm water—also can be hazardous to human health.