The Avengers #40

In "Suddenly... the Sub-Mariner!", the Avengers find themselves in a shocking clash with the enigmatic Sub-Mariner, as a mysterious Cosmic Cube becomes the center of a high-stakes conflict. Written by Roy Thomas and brought to life by Don Heck’s dynamic pencils and George Roussos’s bold inks, this 1967 issue delivers a tense, pivotal moment in the team’s early history—complete with a cover by Heck that captures the intensity of the showdown.