Submarines are larger, more sophisticated and have powerful underwater propulsion. They must travel horizontal distances submerged, require precise control of depth, yet do not descend so deeply, nor need to dive vertically on station. Their primary means of controlling depth are thus their diving planes (hydroplanes in UK), in combination with forward motion. At the surface the ballast tanks are emptied to give positive buoyancy. When diving, the tanks are partially flooded to achieve neutral buoyancy. The planes are then adjusted together to drive the hull downwards, whilst still level. For a steeper dive, the stern planes may be reversed and used to pitch the hull downwards.