Supercavitation

Projectile (red) travelling through the water in a bubble of its own creation. The flattened nose creates areas of very low pressure that result in bubbles (cavities). When these merge into a bubble that encloses the projectile, it is called supercavitation. Larger objects can use supercavitation but they must augment the pressure drop with gas expelled at the nose (e.g. rocket-powered torpedos).