That is a ballast tank used to alter the angle of the submarine up or down

A ballast tank is one of several external or internal tanks fitted in submarines, which allow the boat to submerge when filled with water.

ballast water tank in a tanker which is completely separated from oil cargoes and fuel oil system and is permanently allocated to the carriage of ballast; this arrangement reduces the risk of pollution when deballasting; commonly abbreviated as SBT

Ballast tank

Clean ballast refers to the ballast stored in a tank not permanently utilized for ballast (see segregated ballast below), but has been cleaned prior to use so clean ballast stored in it, if discharged from a stationary ship into calm water on a clear day will produce no evidence of oil, sludge or emulsion on or under the surface of the water or on adjoining shorelines.

Segregated ballast refers to ballast stored in tanks which are permanently utilized for the carriage of ballast, and are completely separated from the cargo oil and oil fuel system.