Supercavitating vehicles and weapons: By traveling inside drag-cutting gas pockets, new subsea systems can move much faster underwater than their conventional counterparts on the same amount of energy (via hmgovt on MeFi. Some interesting comments and discussion developed, including discussion of GI Joe's SHARC. rodii over there kicked in another article.).
Interestingly, the Soviets developed this tech long ago and using it may have been what destroyed the Kursk.