Secret 1: Go to level 4 of the human campaign and play it. Walk up to the jump pad at the left of the battlefield. Jump it and wait until you land. Then walk a bit to the right and shoot up in the air with your sniper!
Secret 2: Go to the first level,after the missile strikes go back to the beginning.
Secret 3: Go to level: Frostbite and try to climb up the iceberg totally. Then jump,you will be launched in the sky and it will reveal a secret.
Secret 4: Go to level: The facility and go to the down left of the map. you will see a few trees. Shoot them as long as you can. It will take a while but it sure does work!
