Holy! This is amazing. From the perfectly judged opening, to the gorgeous graphics (I seriously think this is the best looking game I've played so far), to the conceit, the mysteries of the behaviour of the caves... Man. Like, I'm trying to come up with constructive (ie negative) feedback, and all I've got is "there are better trail renderers out there"...
(But: I stopped playing to make this comment. I kind of don't want to continue. I love the world, the mood, so much I kind of feel like understanding it will ruin it.)