Decorah crater is a buried possible impact crater on the east side of Decorah, Iowa, formed about 470 million years ago when a meteor roughly 200 meters wide struck the area. About 3.5 miles across, it is hidden beneath Winneshiek Shale below the Upper Iowa River. The impact, estimated at about 1,000 megatons of TNT, depressed underlying Ordovician and Cambrian bedrock but apparently did not reach the Earth's mantle. The site is significant not only for the impact itself but also for sediments later preserved inside it, which contain unusual Middle Ordovician life, including Pentecopterus decorahensis, a nearly 6-foot-long eurypterid regarded as the largest known predator of its time.