In an interview in July 1977 with 'The New York Times,' Curd Jürgens (who played the German submarine commander Capt. Von Stolberg), said: "This was an important picture for me because it was the first film after the war in which a German officer was not interpreted as a freak." Other reviews commented that the film was notable because the clash between the captains was not portrayed as black and white or good and evil.
One of the technical consultants working on this movie was Albert Beck, a former German U-Boat submariner, who advised on the submarine scenes.
In real life, the destroyer USS Borie (DD-215) rammed U-boat U-405 and sank it. The Borie was severely damaged, could not be salvaged and sank the next day. The crew was rescued by another vessel.
Two endings were shot --- In one, both commanders die; in the other, a third vessel rescues them. The final ending was determined by preview results. The "USS Haynes" was in actuality the USS Whitehurst, captained by Walter R. Smith, who received a "technical advisor" credit and can be seen playing the ship's chief engineer. Eva Novak can be seen in a photo as the wife of Von Stolberg (Curd Jürgens).
The American ship used in this movie was the destroyer escort USS Whitehurst (DE 634). It was sunk as a target for a new torpedo on 28 April 1971.
Darryl F. Zanuck: (at around 62 mins) The Captain calls a conference of all officers and chiefs in the wardroom. At the left rear of the wardroom, there is a "Chief" in blue clothing with a blue Mae-West life preserver --- it is Darryl F. Zanuck, who at the time was the head of 20th Century-Fox Pictures.
Eva Novak: As the wife of German submarine commander Von Stolberg (Curd Jürgens). Though Novak neither appears in the live-action of the movie itself nor is listed in the cast credits, she is seen in a photograph appearing as Mrs Stolberg.