The Abyss

The Abyss is a science fiction film from 1989, directed by James Cameron, starring Ed Harris, Mary Elizabeth Mastrantonio, and Michael Biehn. There is a cinema version (143 minutes) and a Director's Cut version (171 minutes).

Attention: Wikipedia contains spoilers

The story is about an underwater oil drilling rig that is send by the US military to examine a lost submarine, which leads to an international crisis bordering on a new world war. It turns out that at the bottom of the name-giving abyss near the crash site of the submarine, an underwater city of water-dwelling aliens is located. A Navy Seal, suffering from pressure sickness, turns paranoid and tries to destroy the city with a nuclear weapon from the crashed submarine. The aliens, in turn, threaten to destroy all coastal regions of the world by giant tsunamis.