Gregor the Overlander - The 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.