WARNING: Micro-spoiler to follow. The thing I mainly recall about Contact is that a message is sent by somehow *altering* the digits of pi. So that it was still pi, but with different digits. This struck me as so mathematically absurd that it was hard to swallow the rest of the book. But it was a very good book anyway. --Dan
Carl Sagan's novel _Contact_ is mostly about communications with extraterrestrials. But a far more interesting subplot is about a very different sort of communication. The aliens point out to mankind that if you calculate pi in base eleven, near the beginning there's a square number of consecutive digits that are all 0 or 1, and when plotted as a square they give a picture of a circle. A little further into the number there are reams of meaningful binary data.