Subterranean. Ben Hood Thrillers, #15

Deep in the cold dark limestone caverns of the South Australian coastline, drugs worth millions of dollars are being smuggled inland from ocean caves. An experienced diving instructor becomes suspicious and her mother is concerned enough to hire ex police detective Ben Hood to watch her. Ben enrols in her cave diving course which ultimately leads them both into a dark sinister world where deception and death are frequent stalkers.
Ben is forced to deal with ruthless businessmen, professional killers both male and female and a stunning police detective who shows an intense interest in his activities and takes him to limits that have nothing whatsoever to do with the subterranean world.