To John Tregard, the mysterious Shelbourne was an implacable enemy - why did the volcanic rumblings around Mercury Island kill animals and men... but never imperil Shelbourne? What was the secret of the Glory Hole - where a diver died instantly? The goal for both was diamonds under the sea. To get them, men have killed and will kill again. Tregard could handle that - but Shelbourne would not be brought to a clean, head-on confrontation...