The vanished - Celia Rees

At the start it was just stories. Stories of plague graves, vanished
children and hidden steps leading to a festering underworld.
But when another child goes missing, Fraser and Cassie wonder if
there's some truth in the tales. The dark tunnels running under the
city are real enough. Then there's Billy, who promises to lead
them to a decaying kingdom far more terrifying than anyone could
imagine...
Highly-acclaimed Celia Rees writes superb supernatural horror novels
for teenagers. The Vanished is one of the best.