Being constructed just prior to the Invasion, its builders had to evacuate before before it was completed.
Reactor is a symmetrical map, and takes place in a C-shaped valley, around a circular building. At opposite ends of the C shape, the two team's bases can be found. Each team has a Warthog and a Mongoose. In the center of the valley there is a small building behind which the Ghost spawns. In this building, there is a neutral Rocket Launcher.
The circular building around which the map revolves contains the titular reactor. Above this reactor, hovering in mid-air, there is an Overshield. To reach this overshield, the player must wait until the reactor rods raise out of the water, and then use them as stepping stones to reach the power-up. There are several entrances to this main room all the way around the building. In addition, the building has a second level, which is made up of an indoor corridor, and a walkway running all the way around the outside of the building. Upon this walkway, at the center of the map, the Sniper Rifle spawns.