The Unknown (Animorphs #14)

There's a new rumor in town. Someone has discovered an item that proves life on other planets exists, and they've been hiding it on a base called Zone 91, the most secret place on Earth. Cassie and the other Animorphs already know about life on other planets. Too well. Their enemies the Yeerks will try to access Zone 91, to find out if what's there will threaten their mission to conquer to the planet. So the Animorphs decide to pay Zone 91 and the Yeerks a little visit. But what they discover is not at all what they expect.