Final preparations are under way for the June 9 launch of the international Aquarius/SAC-D observatory. The mission’s primary instrument, Aquarius, will study interactions between ocean circulation, the water cycle and climate by measuring ocean surface salinity. On average, seawater in the world’s oceans has a salinity of about 3.5% (35 g/L). In addition to Aquarius, the observatory carries seven other instruments that will collect environmental data for a wide range of applications, including studies of natural hazards, air quality, land processes and epidemiology. Although the vast majority of seawater has a salinity of between 3.1% and 3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with fresh water runoff from river mouths or near melting glaciers, seawater can be substantially less saline. The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and river inflow, and confined circulation result in unusually salty water. The salinity in isolated bodies of water (for example, the Dead Sea) can be considerably greater still.