Nearly 400 feet beneath the freezing Sea of Okhotsk in the far east Soviet territorial waters, two divers clung to their lives through the umbilical cords that pumped warm water into their diving suits.

As the men scoured the ocean floor, they kept looking for a 5-inch-diameter cable near their submarine’s underwater skis.

However, not even they knew precisely what they were doing or why, as their mission was so shrouded in secrecy that the sailors were told they were testing ocean engineering equipment.

Little did they know that the submarine was conducting a series of spy missions to tap into Soviet communication cables between military bases, and was right on its way to becoming the most decorated submarine in United States Navy history…