I was hiking in Yellowstone with the DW and FIL when we heard this rumble. We stopped and listened as it got louder and louder. We figured out what the noise was when we spotted a herd of buffalo that just popped over the rise on a dead run straight at us. We were in the open with no place to go. About fifty or so split and ran around us and just stopped. Yep, stopped with us in the middle of the herd. Since we had previously owned a dairy farm we were use to close contact with cattle but this was very uncomfortable! We very slowly and causally sorted our way through herd and walked away with some great stories.