Gregor The Overlander by Suzanne Collins

When Gregor falls through a grate in the laundry room of his apartment building, he hurtles into the dark Underland, where spiders, rats and cockroaches co-exist uneasily with humans. This world is on the brink of war, and Gregor’s arrival is no accident. A prophecy foretells that Gregor has a role to play in the Underland’s uncertain future.

But Gregor wants no part of it—until he realises 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 forever.