Supercritical Water in Deep Sea Vent.

Andrea Koschinsky of the International University in Bremen, Germany and colleagues discovered a supercritical hydrothermal vent on the Atlantic seabed:

Scientists working in the southern Atlantic Ocean have found a 407 °C hydrothermal vent, the hottest yet known on an ocean floor. Although only 5 °C hotter than the previous deep-sea high of 402 °C, recorded in the Pacific Ocean, the new hotspot bumps seawater into the strange state of being a supercritical fluid.

Such fluids can diffuse through solids a bit like a gas and dissolve materials more like a liquid. In industrial applications, supercritical carbon dioxide and water are used as solvents thanks to these unusual properties. On the ocean floor, supercritical seawater could dissolve and transport minerals from the surrounding rocks differently than at other hot vents.

http://www.nature.com/news/2006/060522/full/060522-15.html