Hmmm, I just realized I've never discovered any special properties or secrets that deal with the giant ill-looking turtle in the Turtle Cave. Does he have any purpose other than decoration in the game? A quick search of the forum indicated no one had mentioned it before. I wonder if I'm missing something obvious.
I completed the game without doing anything with the turtle, although I seem to remember some sort of quest with the turtles, or perhaps I'm imagining it.
That turtle must be there for a reason; it would be a crime to just leave him there like for the rest of the game without being able to help him at all.
Now here's what we know; you can't kill him, there's no way behind him, there's no way under him, there's no way into him (eww... Okay, you can go into a whale, so this isn't as far fetched as it sounds in Aquaria.) and you can't interact with him really. So... I don't know what he does.
Maybe Li does something? Fly Li over there and see what he does.
Also try nature form / sun form.
Hmmmm...so we don't actually know anything about it? Maybe it is just an inert part of Aquaria, like the giant snails in Open Waters.
I'm pretty sure I've been there with Li, though I don't think I ever did anything with Nature Form or Sun Form.