A hypothesised perpetual fountain where a long, narrow heat-conducting pipe inserted vertically through a region of ocean where warm, salty water overlies colder, fresher (and therefore denser) water. Water pumped upwards through the pipe would reach the same temperature as the surroundings at the same level (by conduction of heat through the wall of the pipe), while it remained fresher and therefore lighter. A fountain started thusly (in either direction) will continue to flow so long as there is a vertical gradient of salinity to supply potential energy. The idea was first advanced by Stommel et al. (1956) and is discussed in Turner (1973).