From Halopedia, the Halo wiki
Engineering is a room/area on a UNSC ship that provides access to the engines that move and support the ship as well as other vital parts. It is fair to assume that most of a ship's vital systems are maintained here. It is also fair to assume that a certain level of clearance is needed to access the Engineering area.
A standard Halcyon-class cruiser has its engineering section in the middle of the ship. The section covers nearly the entire width of the ship and is the largest single room on board, excluding the hangar bay. It houses the four fusion reactors that are part of the ship's fusion drive system. The engine's fusion drives are located on the third floor of engineering and can be accessed by removing the coverings using control panels.
At the end of the Battle of Installation 04, the Master Chief overloaded the UNSC Pillar of Autumn's engines by destabilizing each of the engine cores with explosives, thus destroying Installation 04. There was a corridor leading directly to an elevator on the third floor of the Pillar of Autumn.
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