Rain and winds buffeted the northwestern Scottish coast as Zachary Killingback inspected a rock stuck in the mud. It wasn’t just any old stone: Weighing in at nearly a quarter of a million tons and measuring longer than a jumbo jet, the boulder had careened to its position some 1.2 billion years ago, which means it may be the oldest rockfall yet found on land.

Killingback, a master’s student at England’s Durham University at the time, wanted to know just what happened in the few catastrophic seconds when the humongous boulder gave way. Rocks have fallen off cliffs ever since Earth was cool enough for rock to form, but few ancient rockfalls have been found in the geologic record. This one