Man-made radiation in the atmosphere comes from primarily two sources: nuclear weapons testing and leakage from nuclear reactors, the latter mostly a result of nuclear plant accidents. After the United States invented the atomic and hydrogen bombs, there was extensive testing from 1945 through 1968. Over three hundred warheads were detonated during that time, mostly in desert regions and on small Pacific islands. The result was huge quantities of radioactive isotopes being spewed into the air, including carbon-14, strontium-90, iodine-131, and cesium-137. While precautions were taken by the military so that no one was killed in the initial blasts, radioactivity in the air traveled on wind currents and poisoned areas hundreds of miles from the tests. For instance, two days after a May 1953 test in Nevada, radioactive hail—some stones the size of tennis balls—fell in Washington, D.C. Later, the United States tested nuclear weapons underground in an effort to curtail this air pollution, but, of course, the radioactive wastes of subterranean nuclear explosions can easily make their way into underground water supplies. Other nations, too, have conducted nuclear weapons tests over the years, contributing to the problem.