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.