The Last Continent by Edmund Cooper, cover by Chris Foss (1971)
(Source: the25centpaperback.blogspot.com)
It was just like a fish. If it stayed very still, no one could see it. And then, this was the whole, I remember seeing it very clearly in my head, that, you think you’re looking at a rock just as if you were under the sea and then suddenly something moves and you realize that it’s an object.
Chris Foss on Pirate Ship design for Jodorowsky’s Dune