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.
The full-scale model was little more than a hollow shell with an engine, and could be used only in calm waters. While it was being filmed in rougher weather, it cracked in two and sank. It was later recovered, patched with wood planks and used for the final shots.