As soon as the level begins, turn around. You should see a tunnel leading off to the area you just came from. Head down that tunnel, as if you were going to get the Sputnik Skull. Then, when you come out of the tunnel, head towards your left, towards a wall. You will notice (if you are in the right spot) that there are lights on the ground, and in front of one of those lights, one on the very end, there is a tree. Get on the light, and jump onto the tree's lowest branches. After you land on the first, jump up to the top branch, and then go as far as you can on the branch. This may require some practice to get right. Next, turn towards the wall that the lights are shining on, and jump on top of it (you may need to do a crouch jump). If done correctly, when you walk forward a bit more, you should see the area where you were when you began the level. Now you can go wander the mountains of Quarantine Zone. Note that it is also possible with the Sputnik Skull to grenade jump to the cliffs to the left of the wall with lights.
When you reach the Gondola, go to the middle of the elevator, then wait until you enter the tunnel system. Jump onto the first platform where Flood attack from, then go to the back of it and jump carefully onto the bar holding the platform up, and walk down onto the strip that the Gondola hovers above. If you jump down carefully, you can reach the dark area at the very bottom of the tunnel.
Do the same as the above, except do not jump off the Gondola until you go past the first tunnel system and pass through the first large bulkhead. Then jump onto the platform when you enter that tunnel right after the dark room and jump onto the large bar that the Gondola travels near and to the right of, and you should be right in front of another large bulkhead. However, this bulkhead lacks the actual bulkhead, and is instead just the surrounding circular frame. Jump onto it while going left slightly, and you should slide onto the bottom of the door, then fall off of it, and you should be in a large corridor. There is a secret room in the corridor that can be found.
Having the Sputnik Skull activated, go to the level Quarantine Zone on easy, and play until you are in the Spectre where there is a Flood form that has a Rocket Launcher, shortly after encountering a large valley with Flood-controlled Wraiths attacking the downed Sentinel Constructor Factory. Allow the Flood to fire one rocket at you. When the rocket hits, you will be thrown in the air, and you will land on the outside of the level without getting killed. This may take a few tries.
Travel through the level to the Sputnik Skull, pick it up, then return to your original spawn point at the level's start and look around. On the wall you should see a white spot. Go towards it. Grenade jump and you should reach an area with a giant piece of metal sticking out of the ground. Keep walking, and you should reach an area where you can see where you started in addition to the rest of the surrounding area.