Shield world

Shield World is a solid Dyson Sphere within Slipspace built by the Forerunner. It contained an environment suitable for Earth life. The only way in was a small portal that constituted the only real-space component of Shield World. This portal was located inside the Core Room Antechamber within Onyx.

Dr. Catherine Halsey theorized that Shield World was intended by the Forerunner as a form of "bomb shelter," a place in which to ride out the devastating Halo pulse. As demonstrated in 2552, access to Shield World was only possible upon the imminent activation of the Halos.

Curiously, when Halsey and company fled into Shield World during the last stages of the Battle of Onyx, it appeared to be deserted, implying that the Forerunners did not utilize it when the Halos were last activated.

Trivia

 * Onyx was referred to as "the Shield" of the Forerunner, while the Halos were "the Sword." This echoes Cortana's line on the Halo 3 Announcement Trailer, "I am your shield, I am your sword," but so far Bungie has said nothing about a correlation between Cortana's use of the word "Shield" and Onyx's "Shield World."