Entombed — Christopher Krovatin

Severed limbs. Rotting flesh. Popping bones. Best friends Ian, Kendra, and PJ have seen it all, and they've spent the last few months risking their lives to rid the world of zombies. Now they face their biggest challenge yet: tracking down the vengeful ex-Gravedigger who is holding their mentor hostage in an abandoned underground city. Deep in the menacing caves of Kudus, time is running out for O'Dea, whose powers can be used to unlock a deadly curse.

If that weren't enough, all the kids are keeping things from each other -- secrets that threaten to break up the group and thwart their efforts to destroy the gruesome zombies that are crawling all over Kudus. Will their friendship survive this harrowing underground mission? Will they escape Kudus with their lives?