The cast was deliberately kept indoors continually during the shooting period in order to look as pale as a real submarine crew would on a mission at sea.
Scenes were shot in sequence so that the growth of beards would be entirely natural, although a few shots had to be redone later on with false hair.
The bulk of the film's $15 million budget was spent on constructing U-boats. Specifications for the original Type VII-C U-boat were found at the Chicago Museum of Science and Industry. The plans were taken to the original builder of the subs, who was commissioned to build a full-sized, sea-going replica, their first such assignment since the war ended. A second full-sized model was built for interior filming.
While originally filmed in German, all of the major actors could speak English. When the movie was dubbed into English for USA and UK distribution, all of the principal actors, with the exception of Martin Semmelrogge, dubbed their own voices into English. Semmelrogge dubbed himself for the director's cut in 1997.
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