[TOMT] Children's book about a kid who finds a hole behind his laundry machine that leads to an underground society.

I honestly don't remember much else. I remember there's a part where one of the inhabitants of the below-world teaches this kid to see in pitch darkness, or maybe he Tophs him with hearing stuff or echolocation.

I think it was a series? It wasn't, like, a little kid's book, it didn't have any pictures. Think harry potter or eragon-ish.