Record hotspot found underwater
Scientists working in the southern Atlantic Ocean have found a 407 °C hydrothermal vent, the hottest yet known on an ocean floor.
(if somebody has acces to the full text... please let me know)
Well, this is hot!
but if we go ahead, these hotspots are at 3000 meters under the sea, that means about 300 bar (4350 psi) of pressure...
So water in these condition is a Supercritical Fluid. (Critical temperature and pressure of water,374°C; 217.7 atm, 3147 psi).
So we have a source of natural Supercritical Water!