On Level 5 while escorting the mammoth, at a certain part of the mission, the level is NOT rendered and when I have to leave the mammoth to continue on foot I fall through what should be the floor and die. and when the game reloads its the same thing, i can't proceed with the single player campaign because i cant even get off the mammoth or i fall to my death. any input or information would be greatly appreciated because i would love to be able to finish the campaign and see how the story progresses.