Tunnels Series

The Tunnels series by Roderick Gordon and Brian Williams is a six book series published between 2005 and 2013. The series is categorized as "subterranean fiction." The premise is that there's a world underground that is discovered by a 14 year old boy. I haven't read it in a while, but I enjoyed the books. The books are: Tunnels, Deeper, Freefall, Closer, Spiral, and Terminal.