| An experimental type of naval weapon that has definitely been developed by Russia, and most likely by a few other world powers as well. The Russian version is called the Shkval, or Squall by some American news agencies.
The concept takes advantage of the traditionally hazardous phenomenon of cavitation: When an object moves quickly under water, it can create gaseous packets of water vapor in its wake, not unlike the sonic boom of fast aircraft. This process in undesirable because it is noisy and because the bubbles can quickly scour your metal surfaces if you create enough of them.
The Russians took advantage of cavitation by creating a torpedo with a blunt head that was driven by a rocket. By running the torpedo over speeds of 180k/h, the blunt head causes supercavitation to occur: One large packet of gas is induced that encloses all but the very front of the weapon. This causes a massive decrease in drag, which allows the torpedo to carry a respectable payload at a high speed (The Shkval itself runs at over 200 knots).
The Shkval has a couple of important drawbacks:
- Once launched, it is impossible to steer. All design so far went into getting it stable enough to just go straight, and steering is something that will have to wait for future revisions.
- You still need to keep the propeller on it to get it a good distance away from your submarine, so that when you fire the rocket, you won't end up hurting yourself.
In August of 2000, the Russian sub Kursk sank at sea with all hands lost. It is highly probable, but not yet confirmed by the Russians, that it sunk when a test of the Shkval went wrong. Most likely, the rocket ignited when the torpedo was still in the tube. | 
