Animorphs (Quality): The Unknown (Series #14) (Paperback)

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. Having learned that someone has discovered proof of alien life and hidden the proof on the top-secret base called Zone 91, Cassie and the Animorphs struggle to keep the evil Yeerks away from the place.