This is a second opening to the cave, which has been filled in with boulders. The story I heard was that the Americans blasted these boulders into this opening in case Japanese soldiers were hiding in the cave (this is a part of the island where there were many bloody raids by Japanese after Saipan fell to the Americans). Apparently, if this story is true, the Americans did not know about the entry where John was standing in the previous photo.
I can only imagine the terror of the American G.I.'s as they searched for these hiding places, not knowing whether an enemy soldier was lurking behind the next tree or inside the next hole in a rock ready to fire. I can also only imagine the terror felt by any Japanese people who may have been hiding in here as the Americans blasted this hole closed -- and then the relief as the Americans left, apparently without finding the other entry (and exit). Of course, this is all conjecture: I don't know if anyone was hiding in here. It seems somewhat likely to me, however, that they may have been.