A Mess Hall is a room that provides food and drinks for soldiers. Most, if not all, UNSC ships have one. Rather than having on-board cooks, by the 2500s, most vessels have food dispensers instead; however, this results in a smaller choice of nourishment.
In September of 2552, the UNSC Pillar of Autumn was attacked by the Covenant near Installation 04. The Marines surrounded the Mess Hall, stopping the Covenant boarding parties from reaching the bridge. The same mess hall is encountered in the level, The Maw when you fight a pair of Hunters and some Spec Ops Elites and Grunts. This is also the area where a player gets his or her first grenade, a Plasma Grenade, in higher difficulties.