Internals of the LZW and LZ77 and GZIP file formats. I had a rough idea of how this worked, but this is a really good nuts and bolts description.
Grahams Number is so large you can’t even come close to representing it in expoential notation, even if you use every single nook and cranny of the universe available to write the value down. But we know the decimal digits it ends with . . .