"Heavy water is water containing a higher-than-normal proportion of the hydrogen isotope deuterium, either as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO. Physically and chemically, it resembles water, H2O; in water, the deuterium-to-hydrogen ratio is about 156ppm, (see Vienna Standard Mean Ocean Water). Heavy water is water that was highly enriched in deuterium, up to as much as 100% D2O. The isotopic substitution with deuterium alters the bond energy of the water's hydrogen-oxygen bond, altering the physical, chemical, and, especially, the biological properties of the pure, or highly-enriched, substance to a degree greater than is found in most isotope-substituted chemical compounds. Pure heavy water is not radioactive. It is about 11% denser than water, but otherwise, is physically very similar to water." Relatively pure heavy water was produced in 1933, soon after the discovery of deuterium, the stable heavy isotope of hydrogen.