In the Salton Sea, an underwater earthquake causes a crevice to open, releasing prehistoric giant mollusks. A rescue training parachute jump is conducted, but the patrol boat sent to pick up the jumper finds only a floating parachute. One sailor dives in but disappears. The other sailor screams in terror as something rises from the water.
When the patrol boat does not answer radio calls, Lt. Cmdr. John "Twill" Twillinger (Tim Holt) takes a rescue party on a second patrol boat to investigate. They find the deserted patrol boat with a strange slime on it, then the jumper's body floats to the surface, blackened and drained of bodily fluids. Twill takes the slime to the base lab for analysis, where he teams up with recently widowed Gail MacKenzie (Audrey Dalton) and Dr. Jess Rogers (Hans Conried).
A young couple disappear after swimming. U.S. Navy divers investigate and discover a giant egg and the body of one of the victims. The divers are attacked by a giant mollusk, which kills one of the divers. The mollusk attacks the boat but Twill stabs it in the eye with a grappling hook. The egg is taken to the U.S. Navy lab for study and kept under temperature control to prevent it from hatching.
The mollusks escape into the irrigation canal system, attacking livestock, a lock keeper, a trysting couple and others. Navy divers locate the group of mollusks in the canal system, and use explosives to destroy them.
In the meantime, Gail is at the lab with young daughter Sandy (Mimi Gibson). Worried about the lab rabbits being cold, Sandy surreptitiously turns up the temperature control. Twill calls the lab and gets no answer. He arrives and finds that the hatched mollusk has Gail and Sandy cornered in a closet. He fights it with lab chemicals and a CO2 fire extinguisher until other Navy personnel arrive and gun down the mollusk.
What was released from the crevice?
mollusks