Ways to Avoid Losing Oxygen in a Submarine

In 1940, German U-boat commander Otto Kretschmer's submarine was under siege for hours and his crew grew tremendously stressed. In order to preserve oxygen, they had to find ways to stay calm.

from Videos | Smithsonian http://www.smithsonianmag.com/videos/category/history/ways-to-avoid-losing-oxygen-in-a-submarine/
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