 The vents which are used to allow water to enter the submarine's main ballast tanks when it submerges are the main vents and it is air escaping through these that accounts for the spray sometimes seen when submarines dive.To get a submarine to surface, the main vents in the top of the tank are closed and high-pressure compressed air is blown into the ballast tanks, forcing the water out through the flood ports at the bottom of the tank.This increases the vessel's displacement.