At the Junkyard level, shoot down the green Pizza Boy statue (near the Old Warplane and crane). His pizza box will fall near the vertical crusher and form a ramp. Go to the ramp and line yourself up to the crusher. When the crusher falls, use the ramp. When on top of the crusher, there will be a building you can drive through to get some weapons. Off the side of that will be a rotating black cube. Jump off to the black cube to unlock Warhawk's Rooftop level in multi-player mode.