Shield world

Shield World is a complete 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. In Halo 3, these circumstances were revealed to be the result of the betrayal of the Forerunner AI Mendicant Bias, whose defection to the Flood forced the Forerunners to abandon their plans to evacuate into the shield installations.

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."
 * A "shield installation" is mentioned in the cutscene between the levels "The Ark" and "The Covenant" in Halo 3. 343 Guilty Spark states that he had hypothesized that the Ark might be located within such an installation, but was proven wrong by the discovery of its location outside the galactic rim.
 * At the end of Halo 3, it is hypothesized that the Chief and Cortana, along with the rear of the Forward Unto Dawn, drifted to one of these installations. You can see the Forerunner symbols that are found on the top of the Halo rings.