I discovered it on Bo1. I tried it out because I love zombies and zombie games. I could hardly get past round one, so it frustrated me and I quit playing. With no manual or instructions on how to play I gave up. I got Bo2, played MP for the first couple months and my love for zombies made me try the zombie mode again.
This time I put on my try hard boots an just failed until I learned how the zombies "system" operated. Once I figured it out, I was hooked instantly. I honestly have RARELY gone back to MP since. Then I begin wishing I had played all those awesome maps on Bo1. So I've been playing both bo1 & bo2 lately.
LOVE THIS MODE! :D