Follow the path along the bottom of the crag. Once you arrive at the location, you should find the 'whale's mouth'. The only way to find the cache is to take the same Journey that Jonah did. It's not as easy as it sounds to get swallowed though, and requires a degree of 'intestinal fortitude', so care must be taken lest you are spat out.
(This is a physically challenging cache. You should only undertake it once you've done a little research and you understand the risks involved (you should come with whatever equipment you feel is appropriate, but it may be more hindrance than help). The description and name should give you clues. I'd strongly recommend against doing it alone.)
Many of the logs below give a lot of clues, so if you want to experience the full challenge, don't read them until you've completed it!