Hollow mountain - Thomas Mogford

At the heart of Gibraltar lies the Rock. At the heart of the Rock lies darkness.
The late-morning sun beats down on the Rock of Gibraltar as bored tourists photograph the Barbary Apes. A child's scream pierces the silence as she sees a monkey cradling a macabre trophy. A man's severed arm.
In the narrow streets of the Old Town below, lawyer Spike Sanguinetti's friend and colleague is critically injured in a mysterious hit-and-run. Spike must drop everything and return home to Gibraltar, where he is drawn into a case defending a ruthless salvage company hunting for treasure in the Straits. As Spike battles to save his business, he realises that his investigations have triggered a terrifying sequence of events, and that everything he holds dear is under threat.