Ocean cold as water at greater depths. Below the thermocline, water is very cold, ranging from −1 °C to 3 °C. Because this deep and cold layer contains the bulk of ocean water, the average temperature of the world ocean is 3.9 °C. If a zone undergoes dramatic changes in salinity with depth, it contains a halocline. If a zone undergoes a strong, vertical chemistry gradient with depth, it contains a chemocline. The halocline often coincides with the thermocline, and the combination produces a pronounced pycnocline. The deepest point in the ocean is the Mariana Trench, located in the Pacific Ocean
World Ocean Database Project released approximately every four years, 1998, 2001, and 2005. The most recent World Ocean Database series, WOD09, was released in September 2009. The WOD09 has more than 9 million temperature profiles and 3.6 million salinity profiles. The table shows a comparison of the number of stations by instrument type in WOD09 with previous NODC/WDC global ocean databases. Ocean profile, plankton data, and metadata are available in the World Ocean Database for 29 depth-dependent variables (physical and biochemical) and 11 instruments types: Ocean Station Data (OSD), Mechanical Bathythermograph (MBT), Expendable Bathythermograph (XBT), Conductivity, Temperature, Depth (CTD), Undulating Oceanographic Recorder (UOR), Profiling