The underground

Rachel, the other Animorphs, and Ax have discovered a new super weapon against the Yeerks. It's easy to get. It's lightweight. It's instant oatmeal. It doesn't actually destroy the aliens, but its effect on them is so powerful that the kids know they have to try to use it. To help slow down the invasion. It means another trip down to the Yeerk pool. And the possibility of not being able to get back up to the surface.