After you clear the landing zone and get dropped into the battle, you will get to the top of a small hill on which you will fight off Chimera and some of Chameleons. After you clear this area, notice the large boulder with a destroyed greenish colored bomb next to it. Look at the boulder. Notice it has a crack on it. Walk up to the boulder and keep walking. You will walk through it and find yourself in a small area surrounded by various rocks and boulders, all of which you cannot walk through, but can see through when you walk up to them. To get out, walk back through though the boulder you came in from.