The Dust Bowl drought of the 1930s was one of the worst environmental disasters of the Twentieth Century anywhere in the world. Three million people left their farms on the Great Plains during the drought and half a million migrated to other states, almost all to the West. But the Dust Bowl drought was not meteorologically extreme by the standards of the Nineteenth and Twentieth Century's. Indeed the 1856-65 drought may have involved a more severe drop in precipitation. It was the combination of drought and poor land use practice that created the environmental disaster. Much of the Plains had been plowed up in the decades before the 1930s as wheat cropping expanded west. While natural prairie grasses can survive a drought the wheat that was planted could not and, when the precipitation fell, it shriveled and died exposing bare earth to the winds. This was the ultimate cause of the wind erosion and terrible dust storms that hit the Plains in the 1930s. There had never been dust storms like these in prior droughts. In the worst years of the 1930s on as many as a quarter of the days dust reduced visibility to less than a mile. More soil was lost by wind erosion than the Mississippi carried to the sea. Although the numbers are not known, hundreds if not thousands of Plains residents died from 'dust pneumonia', a euphemism for clogging of the lungs with dirt.