Can we go supersonic... underwater?
In air, the speed of sound is about 340 m/s, but in water, it is much higher. There, sound travels at a speed of some 1,500 m/s, corresponding to 5,400 km/h. For this reason alone, supersonic speed is very difficult to obtain in water. Moreover, water also offers much more resistance than air, and even bullets are slowed down after a few metres.
Still, US scientists in 1997 managed to make a bullet briefly break the sound barrier in water. The bullet travelled through a small air pocket, which scientists had created to reduce water drag. Known as supercavitation, the method is used, when torpedoes are launched under water. The air pocket can reduce the drag by 70 %.
A bullet is slowed down fast in water, but the resistance is reduced, if it is surrounded by an air pocket, like in the lower picture.