Gregor the Overlander Book I (Teacher's Edition, Book I in The Underland Chronicles)

Living with his single mother and two sisters, Gregor is left home alone in the family's New York City apartment to watch his baby sister and grandmother. When Gregor's sister falls through a secluded grate in their apartment's basement, he dives in after her, only to fall miles below the earth into the Underland; a world inhabited by humans with near translucent skin, giant, sentient bats, mice, rats, fireflies and bats, and where an escalating conflict between the human city of Regalia and the rats is brewing. At first, Gregor wants no part of the conflict, but when a prophecy reveals more about him than he'd like to believe, Gregor and the Regalians trek across the Underland, hoping to save it from impending doom. --wikipedia