A 1957 Roger Corman film. From Wikipedia:
Attack of the Crab Monsters concerns a second scientific expedition that is sent to a remote Pacific island to discover what happened to the scientists of the first. Unknown to them when they arrive, the island is inhabited by a mating pair of two radiation-mutated intelligent giant crabs that consumed the first expedition. The giants are also slowly undermining the geology of the island, causing it to fall away, piece-by-piece, into the ocean.