I was reading Lary Niven's "The Ringworld Engineers" last night. In the last chapter I read, they were flying the Needle underneath the Ringworld, passing under The Great Ocean, looking at the indentations of the planet maps in the Ringworld's outside surface. When they get to where Mars should be they see no indentation. The characters mention that it must have something to do with the superconductor grid embedded in the Ringworld. I'm totally confused by this. Will this be explained in later chapters? It feels like I've missed an important plot point.
If your answer will contain spoilers for the rest of the book please warn me first.