Filmed in 1942 when British losses in the Battle of the Atlantic reached their peak.
To prepare for the film, John Mills was allowed to go on patrol in one of the submarines. He was determined to know what it felt like when a submarine crash-dived.
Initially telecast in Los Angeles, 3/20/50 on KECA (Channel 7), and in New York City, 9/23/50 on WCBS (Channel 2). Since it had never previously been commercially released in the US, these presentations also constituted its US theatrical premieres.
One of the submarines used for this movie, the P615, was sunk in action a few months later. She was torpedoed by the U123 near Sierra Leone on 4/18/43. All 44 crew members lost their lives.
The filming of the submarine was shared between submarines P614 and P615, with the last digit painted over, leaving "P61", which has led to its being identified as H.M.S. Varangian.