Gregor the Overlander (Underland Chronicles)

When eleven-year-old Gregor falls through a grate in the laundry room of his apartment building, he hurtles into the dark Underland, where spiders, rats and giant cockroaches coexist uneasily with humans. This world is on the brink of war, and Gregor's arrival is no accident. Gregor has a vital role to play in the Underland's uncertain future. Gregor wants no part of it - until he realizes it's the only way to solve the mystery of his father's disappearance. Reluctantly, Gregor embarks on a dangerous adventure that will change both him and the Underland for ever.