Three Days to Live — Robert Charles

Mike Harrigan was scar-faced, a drifter, and something of a woman-hater. With his partner Dan Barton he searched the upper reaches of the Rio Negro in the treacherous rain forests of Brazil, lured by a fortune in uncut emeralds. Behind them rode three killers who believed that they had already found the precious stones. And then fate handed Harrigan not emeralds, but the lives of women, three of them nuns, and trapped them all in a vast series of underground caverns.