A sub-atmospheric pressure desalinating still employs a closed top, open bottom pipe filled with source water to be distilled, such as seawater, having a height greater than the height of a column of seawater that can be supported by the pressure at the bottom of the tank so that a subatmospheric pressure volume is formed at the top. Water from the source is also pumped into the subatmospheric volume and passed through an evaporator which enlarges its surface volume. A small percentage of the water is vaporized and the balance is cooled to provide the heat of vaporization and falls into the top of the seawater column, creating a downward flow. The vapor is drawn from the vacuum and condensed, preferably in a second subatmospheric volume above a column of fresh water.