Natural gamma radiation has cosmic, atmospheric, and terrestrial components. The cosmic and atmospheric components can be highly variable with time, elevation, and prevailing atmospheric conditions. Gamma radioactivity utilized in mineral-resource exploration programs can be primary or secondary. Primary sources include those in deposits of uranium and thorium. Secondary sources include those found in minerals accessory to the principal minerals of interest; examples of secondary sources include uranium in phosphorite and thorium and uranium in placer heavy minerals, such as zircon, monazite, apatite, sphene, and the epidote group. The principal mineral resources in the submarine environment include sand and gravel, phosphorite, heavy minerals, manganese crusts and nodules, and massive sulfides. The radioactivity of marine sediments has been investigated in laboratory and field settings. Radioelements in solution in fresh water and in seawater may be adsorbed onto sediments after their deposition. Organisms may play an important part in concentrating radioelements from solution in seawater.