Manson crater
As the site was covered by a glacial hill fairly recently, no eveidence of the crater can be seen on the surface. Today, the site of the crater is a flat landscape. Below the surface, hidden about  below the surface is a buried structure about  in diameter. It lies under the southeast corner of Pocahontas County and extends under portions of three counties. That an anomalous structure underlaid the area was known from unusual water well drill cuttings in 1912 of deformed rock,  "crystalline clast breccia with a melt matrix" as a later report described it. A research investigation was started in 1955, and it was labeled a "cryptovolcanic structure" (a hypothetical volcanic steam explosion).  Robert S. Dietz did some more investtifatiom and proposed an impact origin in 1959. Nicholas Short found evidence of shocked quartz grains in 1966. This confirmed the impact origin of the structure.