The Avengers #40 – Suddenly… The Sub-Mariner released by Marvel on May 1, 1967
The Scarlet Witch collapses from stress sustained in their most recent battle (last issue). Elsewhere, the Black Widow steals an experimental jet from General “Thunderbolt” Ross.
The Sub-Mariner attacks a submarine which has been destroying underwater wildlife. He then decides to attack the base it came from. The Avengers receive a call from Captain America warning that a dangerous item called the Cosmic Cube has been seen near the base and the Avengers should secure it.
While Quicksilver stays with his recovering sister, the other Avengers accompanied by Hercules journey to the base and battle the Sub-Mariner. Just as the Avengers are on the verge of defeating Namor, Hercules unwittingly mentions the Cosmic Cube to him. The Sub-Mariner (who had located the Cube earlier but was unaware of its abilities) flies away but returns with the Cube and the battle begins again. Namor is beaten, however, when he loses contact with the Cube, and it is lost down a crack in the ground which seals itself after.
Sometime later, the Mole Man finds the Cosmic Cube but throws it away, dismissing it for a useless bauble.