>'going faster than the speed of sound underwater'
Some reports indicate that the VA-111 possibly exceeds speeds of 250 kn (460 km/h; 290 mph), and that work on a 300 kn (560 km/h; 350 mph) version was underway.[7][8]
The high speed is made possible by supercavitation, whereby a gas bubble surrounding the torpedo is created by outward deflection of water by its specially-shaped nose cone and the expansion of gases from its engine and the gas generator in the nose. This minimizes water contact with the torpedo, significantly reducing drag.[2]