Plot summary
An experimental submarine, the Siren II, with an experienced NATO crew, is sent to find out what happened to the missing Siren I. Wick Hayes, the designer of the sub, blames the Contek corporation's modifications to his original design. The Siren II is captained by experienced officer Captain Randolph Phillips. The Siren II traces Siren I's black box to an underwater rift. They are surrounded by a toxic weed, although an on-board scientist says plant life at this depth is impossible. Siren II escapes the weed by reversing the polarity of the hull, although some of the weed gets into the sub. Later, they surface in a cave system where they discover that Contek has been engaged in illegal genetic engineering experiments that have produced a variety of mutant creatures.
Credits
Adapted from Wikipedia contributors, The Rift (1990 film). Section headings and formatting adjusted.