Underwater Bullets at 27,000fps-Supercavitation-Science Movie Today

      The bullet flies underwater in the speed which is several times faster than water's. As a result, the bullet push water away, the water would not refill the space immediately. So the bubble is created. People call the phenomenon "Supercavitation".

       is the use of cavitation effects to create a bubble of gas inside a liquid large enough to encompass an object travelling through the liquid, greatly reducing the skin friction drag on the object and enabling achievement of very high speeds. Current applications are mainly limited to projectiles or very fast torpedoes, and some propellers, but in principle the technique could be extended to include entire vehicles. This phenomenon can also be produced by the very fast strike of the appendages of the crustacean mantis shrimp Odontodactylus scyllarus, that uses it to attack and kill its prey. (Wikipedia)