A spreading mushroom cloud rising majestically into the atmosphere has become a frightful symbol of the atomic age. The cloud itself is both awesome and magnificent. But behind it are the three destructive forces of an atomic explosion- blast, heat and radiation. Fallout is the dust and debris that are pulled up into an atomic fireball and return to earth radioactive. Fallout occurs in two stages: local, the fallout that occurs within the first two weeks, and delayed. Delayed fallout is the radioactive material that is caught up into the earth's atmosphere and returns to earth gradually over a 10 year period. Radiation is always the result of an atomic explosion. When atoms are split, various particles go flying and various by-products are formed. The liberated particles are radiation. Scientists have discovered that 700 rem will probably be fatal to life. The rem from a nuclear attack might reach 10,000 during the first two weeks of fallout. After the two weeks, though, it would be relatively safe. It is for this reason the idea of shelters have gained popularity.