DeepStar Six was one of a trio of late-80s movies, along with The Abyss and Leviathan, set in sea floor habitats miles beneath the surface of the water. In this case, the crew are attempting to install a deep-sea missile platform for the Navy.

Things are going well until the crew uncovers a massive cavern during their excavations. First they send in an AUV to scout around but it dissapears. Then they send in a manned sub and it disappears too. Then a manned outpost is attacked and they suddenly realize that they've unleashed some kind of sea monster from the cavern. The monster eventually gets around to attacking the main station, creating just enough damage that the crew has to go outside to repair it -- where the monster's waiting for them!

DeepStar Six isn't terrible but there are major plot flaws. Actions that are supposed to be seen as unselfishly heroic are actually dangerous to everyone within 500 feet. Water that leaks into the station and that should be close to freezing doesn't pose any thermal challenge whatsoever to the crew. And the monster? He's pretty bad, too. The story would have been fine without him.

Some of the underwater photography is cool but the wide-angle shots of the mini-subs are comically bad. All in all, there's nowhere near enough diving in this movie.