An earthquake under the ocean floor hits a lab on the seabed. As a result, it's umbilical cord to the surface ship is severed and the lab tossed over a sheer underwater cliff. Now it is so deep that conventional divers cannot reach it, so the only hope is a deep water submersible, the Neptune.
Despite the film being about an ocean disaster, it's amazing just how static and dull the movie is. I think the main problem is pacing...not acting, as the actors are generally a very accomplished group of professionals. Too often, the director chose to portray the film in the least exciting manner....and I had to really struggle to pay attention or even to care.
By the way, although I don't recommend anyone see this movie, scuba divers might enjoy watching some of the diving sequences...especially when the DIve Master is thrashing about in the water like he just got his certification yesterday!