Peter

__Tunnels__ by Roderick Gordon
Will Burrows goes off in search of his father with the help of his friend. They end up in a sinister world in the center of the earth with bounty hunters in pursuit. This is the first book in the series; here are all of the titles in the series:
 * 1) Tunnels
 * 2) Deeper
 * 3) Freefall
 * 4) Closer