The Phantom from 10,000 Leagues (1955)

A marine biologist teams up with a government agent to uncover the truth behind a string of mysterious deaths linked to a rumored sea monster in a remote ocean cove. Their investigation leads them to a marine biology professor engaged in clandestine experiments. Complications arise when international spies seek to pilfer his groundbreaking research. As they delve deeper, they encounter a sinister creature guarding a radioactive light lurking on the ocean floor, forcing them into a perilous confrontation that could have far-reaching consequences.