Routine Exercise - sf68 (Philip E. High)

Naval vessel Torus, a nuclear submarine, embarks on what’s billed as a routine training exercise. 

Mid-exercise, the crew notices bizarre sonar readings. Suddenly, an inexplicable time shift occurs—the submarine emerges in an ancient, prehistoric ocean. 

Crewmembers spot massive, unknown creatures—prehistoric monsters—lurking in the depths. The sub’s weapons systems even register an unknown UFO-like contact, raising alarm and confusion.