Vienna Standard Mean Ocean Water subsurf.

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VSMOW, or Vienna Standard Mean Ocean Water, is an isotopic water standard defined in 1968 by the International Atomic Energy Agency. Despite the extremely misleading phrase "ocean water", VSMOW refers to pure water (H2O) and does not include any salt or other substances usually found in seawater. VSMOW serves as a reference standard for comparing hydrogen and oxygen isotope ratios, mostly in water samples. Very pure, distilled VSMOW water is also used for making high accuracy measurement of water’s physical properties and for defining laboratory standards since it is considered to be representative of “average ocean water”, in effect representing the water content of Earth.

Previously, average ocean water and melted snow were used as reference points. These were further refined in the 1960s by the standardized definition of Standard Mean Ocean Water (SMOW). The U.S. National Bureau of Standards (now the NIST) created physical water standards for global use. However, the physical integrity of the U.S. standards soon came into question.

VSMOW is a recalibration of the original SMOW definition and was created in 1967 by Harmon Craig and other researchers from Scripps Institution of Oceanography who mixed distilled ocean waters collected from different spots around the globe. VSMOW remains one of the major isotopic water benchmarks in use today.