(The shift started at 6:00PM on 10/6/10.) EE was dismounting the water truck & stepped on a rock with his left foot. He lost footing with the left foot & his right foot slipped through the ladder rung, becoming stuck & twisting at the knee. He heard a ""pop"" & felt pain in his knee. An investigation of this incident was completed on 10/20/10, at which time it is being reported.