From Halopedia, the Halo wiki
Go into Forge mode and spawn a set of teleporters. Place the receiving end on one of the far ledges that is near the giant sea of ice. Place another on an easy place to reach. Go through the teleporter and place a respawn point. But be careful, if you go too far, you'll die instantly. Also, if you go into Edit Mode and head upwards you can reach another part of the map, but you will die if you venture too far out. Another way to get out from there is to go back to where you first placed the teleporter. Face the wall and push against it whilst rising up in Forge mode. Then, head towards the right and you should be able to go further up to a snowy area. Go any further than the snowy area, and you will die.
Another simple way is to go inside of one of the bases, to the back, where the glowing cylinders are located behind the glass wall. You can either spawn a new teleporter, or use the one readily available near the wall. Simply spawn a two-way or a receiver node while against the wall, and it should spawn behind it. Drop it and hurriedly get into the teleporter as a Monitor before it falls, as it will disappear. From there you can make floating objects so that one may walk around in these areas.