Near Manson in northwestern Iowa lies a buried impact crater formed about 74 million years ago during the Cretaceous Period, when a large asteroid or comet nucleus struck Earth along the shore of the shallow Western Interior Seaway. Hidden beneath glacial till, the structure is about 38 kilometers across and ranks among North America’s largest known impact events. The collision shattered deep crystalline and younger sedimentary rocks and vaporized limestone, a legacy still reflected in Manson’s unusually soft water. Study of deformed rock, impact interpretations, and shocked quartz eventually confirmed its meteorite origin, while later dating proved it predated the extinction of the dinosaurs by about 10 million years.