I was messing around with my mammoth last night, I managed to get to many places but this one by far is my favourite.
I searched, found nothing so if this is a repost feel free to tear me a new one.
*PS if you move out of the little corner you will get dismounted.
(Big pics)
[Spoiler]
Mount up on your mammoth here.
Your camera will automatically go to this position in a few seconds.
Keep running and jumping forward until you see this.
Keep going, then stop once your inside, and voila.
[/Spoiler]