Sentinel manufacturing facility

Summary
Sentinel Production Facilities are massive floating platforms that produce Sentinel, Enforcer, and Sentinel Major automatons. These facilities float high above the ground to keep them somewhat safe from the majority of Flood if there is ever an outbreak.

During Halo 2, in the level Sacred Icon when the Arbiter reaches the inner side of the Sentinel Wall, there is an opening in the structure that is similar to the opening where he first entered in on from the phantom. In the distance above the Library there are two large floating objects. One of these objects is struck with a massive plasma mortar (possibly fired from the Flood to slow down the forerunner defenses) and crashes down to the ground. In the next level, you spy the same crashed structure being pounded by Wraiths and Scorpions, and fight through it as Sentinels and Flood fight each other. While the appearance inside the structure is badly damaged, some believe it to be a futuristic assembly line, allowing the Monitor to have endless waves of Sentinels to control the Flood. Others, however, have noted that the room resembles the engine room of the Pillar of Autumn, leading to speculation over whether the facility in question was man made.