ation. As the column falis back into the water, a cloud of mist is formed around the base of the column which spreads rapidly. This cloud of mist, called the base surge, contributes to the spread of | the radioactive contamination by moving outward from the base while the colum, which is not a true cloud, falls back through the close-in surge into the water. This expanding doughnut-shaped mist appears to carry some activity deposited in it from the column over a greater area than would otherwise be contaminated. The base surge is readily influenced by winds and travels in the direction of the prevailing wind. An underground burst, described previously, is one in which the center of detonation is below the ground. The mechanism of cloud formation in this case is initiated by the venting of incandescent gases from the fireball directly above the point of detonation. As the gases are released, they carry a large quantity of earth high in the air in the form of a hollow cylindrical column. The material from the crater, much of it contaminated, is thrown out as for a surface burst. As the material in the column cools, the soil particles and entrained air which form the column begin to behave like an aerosol with a density greater than the surrounding air. The colum thus falls down- ward and the finer soil particles attain velocities greater than their terminal velocities in still air. These dust particles spread out radially to form a low dust cloud or base surge similar to that deseribed for an underwater burst. The cloud from an underground burst does not rise as high as for surface or air bursts, and the spread of contamination is thus influenced to a greater extent by lower wind strata. The spread of radioactive contamination is by fall-out from the cloud, the colum, and the base surge. As burst depth below the surface is increased, conditions become more favorable for formation of a base surge. More of the radioactive contamination is deposited locally with increased depth, until in the case of no surface venting, all of the contamination is contained in the volume of ruptured earth a1