Caesium-137 (), or radiocaesium, is a radioactive isotope of caesium that is formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nuclear weapons. Trace quantities also originate from natural fission of uranium-238. It is among the most problematic of the short-to-medium-lifetime fission products. When suddenly released at high temperature, as in the case of the Chernobyl nuclear accident and with atomic bombs explosions, because of the relatively low boiling point (671 °C, 1240 F) of the element, 137Cs is easily volatilized in the atmosphere and transported in the air on very long distances. After the radioactive fallout, it is deposited onto the soil and easily moves and spreads in the environment because of the high water solubility of caesium's most common chemical compounds, which are salts. 137Cs was discovered by Glenn T. Seaborg and Margaret Melhase.