Played a round a couple weeks ago and someone hit a ball into what appeared to be a trench, approximately 1 foot wide, and 2 feet deep. The trench was located in the side of a hill that lead up to the green. There were no markers indicating the hazard, it was simply a giant hole in the ground from which it was impossible for the person to hit out of because his golf club would have had to be below the ground in order to even attempt to strike the ball, let alone make a backswing. We did not know whether the ball rolled in or whether it simply landed in the trench.
What is the correct ruling under these conditions?