Tunnels (Book #1) by Roderick Gordon and Brian Williams

A subterranean fiction novel by British authors Roderick Gordon and Brian Williams. It was initially self-published as The Highfield Mole in 2005, and re-released as Tunnels by The Chicken House in 2007.
 
The story follows Will Burrows, a 14-year-old 'archaeologist', who stumbles upon an underground civilization called The Colony. Will and his friend Chester flee The Colony and set out to find Will's father, in the Deeps, a place even deeper in the Earth than The Colony.
 
Loved. Good condition, Library discard paperback.