This only opens during the mission where you blow up the satellite dish and are rescued via the helicopter. If your a loot fanatic, then you need to loot those boxes while your in that mission, otherwise, you'll be locked out. You can get back on top of it by jumping some rocks near the door where you enter, but you can never get back inside.
Playing on middle level - after killing everyone guarding the bunker entrance, as you get close to the bunker door a symbol of dynamite sticks appears with the "Press E to Interact" comes up.
Press E to put a charge on the door (not the C4 from key 5} and retire behind the concrete traffic wall, crouch and press LMB - voila! explosion and door opens.
As the man said it is a mission quest that enables you to blow the door. The trouble is once you set the C4 all hell will break out. After many tries I finally succeeded by planting mines all around and at the right time jumped on the heli, manning the gun and going nuts. It felt great after all the frustration of MISSION FAILED (Sam died
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