Sentinel manufacturing facility
From Halopedia, the Halo wiki
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. It seems logical that these structures are the Sentinel Production Facilities; especially since they are located over the location on Halo that has the utmost security protocols. The existence of these facilities on Installation 04 is not known.
