Production of "Das Boot" took two years (1979–1981) and was the most expensive German film at the time. Most of the filming was done in one year; to make the appearance of the actors as realistic as possible, scenes were filmed in sequence over the course of the year. This ensured natural growth of beards and hair, increasing skin pallor, and signs of strain on the actors, who had, just like real U-boat men, spent many months in a cramped, unhealthy atmosphere.