Gregor the Overlander - Suzanne Collins

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.