The Extreme (Animorphs, No. 25)

When the evil Yeerks start wreaking havoc at the North Pole, Marco and the other Animorphs must use their special powers to stop them from taking over one of the coldest places on Earth. Original.

Marco, the other Animorphs, and Ax have managed to find out where the Yeerks are planning to build their next ground-based Kandrona. That's a good thing. The location is supposed to be somewhere around the North Pole. That's a bad thing. The Animorphs and Ax know that the Yeerks are a "cold-blooded" species, but this is a little nuts! Who wants to be anywhere near the North Pole without Arctic morphs--and wearing spandex?

Even so, the kids know if the Yeerks succeed with their plan, Earth is pretty much done for. And Marco, the other Animorphs, and Ax aren't quite ready to give up the fight....