What is the Monster? (2002) [pdf]

Oh! It's nice to see the monster group come prowling into my day unexpectedly. I clicked hoping to read something enlightening by a group theorist, but five hours in and no comments yet.

The sporadic simple groups are these things I've heard about but feel mostly like mythical creatures to me. One day I'd love to have a nice, geometric intuition for each, and it seems like this is slowly becoming possible for the monster group!

Anyway, I'd love to hear any stories people have working with the sporadic groups.