Blood Beach
(1980)

Harry (David Huffman), Catherine (Marianna Hill), and Lt. Piantadosi (Otis Young) discover that the basement of a burned-down building that Harry and Catherine used to hang out in when they were kids has become the den for the murderous creature when they find remains of its victims in there. Determined to destroy it, the police set up infrared cameras in the den so they would know when it was back, and also wire the place with explosives so they could blow it up when it did appear.

That night, the creature returns to the den, and it is revealed to be a giant sea cucumber (at least, that’s what I thought it was. The movie never explains what it actually is). Without hesitation, Sgt. Royko (Burt Young) pushes the plunger down on the detonator, and the creature is blown to bits.

Days later, the beach is up and running again. The camera soon shows a woman on the beach sitting with her back turned to her son. When she asks him if he wants a sandwich and he doesn’t reply, she turns around and sees that he has vanished, with a pit formed where he was once sitting. As she calls out for him, more pits slowly start forming in the sand near people in other areas of the beach, which goes on as the credits roll.

Thanks Tornado Dragon!