The location of antimatter storage in the secondary hull of the Enterprise-B
The antimatter storage was a spacecraft section reserved for storing antimatter in the Excelsior-class starships in the late 23rd century. It was located in the lowest decks of the secondary hull, in the vicinity of the antimatter fill port and antimatter generator.
In 2293, the location of this section was labeled in the USS Enterprise-B master systems display on the ship's bridge. (Star Trek Generations)