Oh, so the 600 meter limit isn't something inherently built into the game? I'll note a really weird thing I experienced: according to kerbalmaps.com the deepest part of the Kerbal ocean is 1391 meters, here: 28 54 18 S 276 53 19 E. I'd gone out there with some sort of vague plan to be Kerbal James Cameron, ascending to the bottom of the deepest trench in the ocean. Pretty soon I had 2 or 3 Kerbals treading water there. I sent another Kerbal out there in a ship (I mean, a boat) I had downloaded that somebody made (Hydra speedboat). Ok, I had to cheat on fuel to get all the way down there to that part of the southern hemisphere. Once I got in the right area, I F5 quicksaved. But when I f9 restored, a bizarre thing happened: the boat instantly went to the bottom, but my Kerbal was thrown clear, treading water on the surface. But when I looked down....beneath my feet, as it were.... the Heads Up Display showed the boat as 1.4 kilometers away. That pretty much confirms the accuracy of that depth, 1391 meters, 1.4 kilometers. So I then switched the view to the boat. It immediately began rising to the surface. It didn't explode when it passed the 600 meter mark. I suppose because, if the 600 meter limit is just part of the HL mod, it wasn't present with any of the vehicles I was using at the time. Though at the time I thought the 600 meter limit was built into the game itself, and just figured the game wasn't configured to detect stuff RISING, not descending, past the 600 meter limit. When the boat hit the surface, it hit so hard it shot right out of the water, like a cork that had been released from deep underwater. I've tried to recreate this a few times, unsuccessfully. Which is too bad, because, I can see it as a sort of depth charge, even if I can't get a Kerbal down there, sunken boats to confirm depth....