Teleportation grid

Halo: CE
Forerunners have created this interesting piece of equipment to teleport from place to place. Sometimes you appear in another green teleportation pad, which is called a 2-way pad, or sometimes you land on the ground. This is called a One-way teleportation pad. Covenant forces have been seen coming in and out, and the MJOLNIR armor allows the bearer of the armor to pass through. Little else is known about this, and no one knows if anything but a Spartan and Covenant forces can pass, as only the Covenant and Master Chief have done so.

In Halo 2
The teleportation grid is portrayed very differntley, as an instrument that covers the entire Halo installation and can be used by Monitors to teleport themselves and Reclaimers anywhere on Halo in a mere second. It at first seemed that only 343 Guilty Spark had access to such an instrument, but it was soon found that Gravemind was also able to use it, possibly through possession of 2401 Penitent Tangent, the monitor of Installation 05.