What is the Down Under? Why does Max like it so much?

The down under is Max's private room in the basement of his grandparents' home. Max is portrayed as a quiet, reclusive adolescent with extremely low self-esteem. Before Kevin "Freak" Avery moves into the neighborhood and becomes inseparable friends with Max, Max keeps to himself and does not have any friends.
In chapter two, Max describes his private room called "the down under" and mentions that Grim glued cheap paneling to the concrete walls to make the space livable. Max goes on to mention that he doesn't even care that the crumby paneling tends to buckle away from concrete cellar walls or that the rug smells like "low tide." Max simply enjoys having a space of his own, where he can be alone and avoid his grandparents, Grim and Gram. Max even refers to himself as a "critter" and spends the majority of his time reading comic books in the down under. After becoming friends with Freak, Max stops spending so much time by himself in the down under and embarks on adventures with Freak, riding high on his shoulders.

"The down under" is the basement room belonging to Max, the narrator in Rodman Philbrick's novel Freak the Mighty. It is introduced in the second chapter, titled "Up from the Down Under." This is a room that Max's grandfather, Grim, has built especially for Max. It is small, and the rooms are made out of cheap paneling, but Max likes it nonetheless because it is private. He thinks that if his room were upstairs, his grandmother, Gram, would check on him and bother him more often. It also serves as his refuge from the world around him.
From the beginning, Max lets readers understand that he does not really have friends and that a lot of times when he has to interact with other people—his peers or adults—he ends up getting in trouble. He feels that by isolating himself in "the down under" he is keeping himself safe from having to deal with the people upstairs and on the outside.