The Unknown (Animorphs, Book 14)

Plot

Someone has discovered an item that proves life on other planets exists, and that item is being hidden in Zone 91--the most secret place on Earth. The Animorphs know all too well about that other life and realize that the Yeerks will try to access Zone 91 and threaten their mission. So Cassie, Ax and the others decide to pay Zone 91 and the Yeerks a little visit--but what they discover is not at all what they expect.