Local fallout right|300px|thumb|The fallout plume from 15 megaton surface burst Castle Bravo, 1954."Estimated total (accumulated) dose contours in rads at 96 hours after the BRAVO test explosion" During detonations of devices at ground level (surface burst), below the fallout-free altitude, or in shallow water, heat vaporizes large amounts of earth or water, which is drawn up into the radioactive cloud. This material becomes radioactive when it combines with fission products or other radio-contaminants, or when it is neutron-activated.
Local fallout right|300px|thumb|The fallout plume from 15 megaton surface burst Castle Bravo, 19.54."Estimated total (accumulated) dose contours in rads at 96 hours after the BRAVO test explosion" During detonations of devices at ground level (surface burst), below the, fallout-free altitude, or in shallow water heat vaporizes large amounts of earth or water, which is drawn up into the radioactive cloud This material becomes radioactive when it combines with fission products or other radio-contaminants, or when it is neutron-activated.