Calvin crater is a buried meteorite impact structure in Calvin Township in southwestern Michigan. Measuring about 8.5 kilometers across, it formed roughly 450 million years ago during the Ordovician period. Rather than appearing at the surface, it remains concealed beneath later sediments, about 100 to 400 feet underground. It was identified in 1987 by Randall Milstein of the Michigan Geological Survey while he analyzed data from about 100 test wells in the area. Its importance comes from both its great age and its hidden nature: an ancient impact crater recognized through subsurface geological evidence instead of visible surface features.