Update II: Natural Supercritical Water

I've found this video about the "Natural Supercritical Water"...



You may also enjoy the article of Andrea Koschinsky and Cols. "Hydrothermal venting at pressure-temperature conditions above the critical point of seawater, 5°S on the Mid-Atlantic Ridge".

Supercritical Fuel System: 50-75% Improvement in Engine Fuel Economy

The TSCi gasoline fuel charge enters the cylinder at around 400 °C—compared to about 100 °C for a conventional liquid direct injection fuel charge—at precisely Top Dead Center (TDC, 0° crank angle).
The supercritical charge facilitates short ignition delay and fast combustion, with the energy released focused just on pushing the piston down. The fast combustion minimizes crevice burn and partial combustion near the cylinder walls, and prevents droplet diffusion burn.