From Halopedia, the Halo wiki
A power room is a place on a Halo installation, which is connected to other power rooms placed around the whole ring. A power room on Installation 04 featured a large vertical core and a dome-shaped roof, with large glass windows that run down the side. The energy which powers the installation can be seen pulsating up through the core and then back down through the windows.
The first time a player sees a power room is in Halo: Combat Evolved at the beginning of the level Assault on the Control Room. It is guarded by some grunts that, being the fact that only one was awake at the time, fail to stop the Master Chief from entering the rest of the installation, which in turn, costs them their lives at the hands of the humans. The power room was conveniently placed for Echo 419 to drop off the Master Chief so he could begin his attack, as it was the closest power room to the Control Room.
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