Piranha (1978)

A young couple stumble across an abandoned US Army test site on a mountain, in which is a huge pool. Thinking it's an ordinary swimming pool, they jump in. But this pool is home to the piranha, and the couple are eaten alive. A young woman P.I. is hired by the father of one of the missing kids to find them, and she meets up with an alcoholic outdoorsman who lives on the mountain. The two of them find the test site and drain the pool to see what's in it. As they do they are accosted by Dr. Hoak - the sole resident of the test site - who informs them that the inhabitants of the pool were the products of a gene-splicing experiment called 'Operation Razorteeth', designed to produce a mutant strain of piranha fish for deployment in the Vietnam War against the NVA. The fish could live in cold water and breed at a high rate. Realizing that a children's summer camp and the Lost River Lake Resort downriver are in the piranhas' path, they set out to try to stop it. The piranha are well ahead of them, and they kill several people on their way downstream. When they try to warn the camp director and resort owner of the danger, they are arrested. Thanks to the woman's ingenuity they escape custody and race down to the camp in a state police car to warn them. But the piranha have already struck - and there are others who want to keep the danger of the piranha a secret...