Deeper

Deeper by Roderick Gordon

In Tunnels, boy archaeologist Will Burrows went in search of his missing father - and discovered a sinister subterranean world.
Now, wandering the dark , burnt bowels beneath the Bolony, Will stunbles across the Styx's diabolical palan to exterminate all Topsoilers. Slowly he begins to piece together the plot. But how can Will save all those above from annihilation when his own life is at risk down below - and when his killer sister is still at large?
(From the cover)

Hey did you know that they're making a movie of Tunnels, should be good, but not as good as the book. Be sure to try and read it before the movie comes out.