Two summers ago I was camping in Rocky Mountain NP. Myself and 2 buddies went on a long through hike from a trailhead up top to our campsite at the base (our 4th friend wasn't feeling good and sat it out so he dropped us off). Anyways, we were pretty experienced hikers and had planned accordingly. It was an 18 mile hike that we had dedicated the full day to. The weather was particularly bad that day and there was a super thick fog that obscured most everything and had visibility down to about 20 feet. It looked like some serious horror movie shit. We hadn't had much luck seeing wildlife and we were hoping to see a black bear but we had really only seen moose from a safe distance. Now my friends made fun of me for being overly cautious around moose but I tried to explain to them how they would fuck our shit up if we ever crossed ones path too closely. Plus they just look like evil black cancerous deers with ballsacks on their necks. About 12ish miles into this hike we are heading up a particularly steep part of the trail and the fog had gotten even worse. Visibility was down to 8-10 feet. We begin to hear this noise in the distance behind us that is gradually getting louder. The best way I can describe it was a loud, guttural grunting noise that was absolutely bone chilling. unse-unse-unse repeat. At varying volume levels. Now we didn't know what it was, but it sounded very very large. We didn't know how close it was because of the fog. It still makes the hair on my arms rise every time I think about it. Needless to say we didn't stick around to wait and find out. We took the best trail section at a slow jog until we couldn't hear the noise anymore and then had a very brisk walk all the way back to our campsite.