The Exposed (Animorphs, #27)

The Animorphs and Ax have one true ally in their battle against the Yeerks: the Chee. So, when they discover that the Chee⁠—including their friend, Erek⁠—are in trouble, they have no choice but to help.

But the Chee's problem is much deeper than anything the kids could have imagined. It's hidden in the ocean. So far down that no human has ever traveled there—and survived. And the Animorphs realize what will happen if Visser Three discovers the valuable secrets of the Chee. One of which is the Animorphs. . . .