This hull opening indicator light panel, also called the “Christmas Tree,” shows the status of hull openings, vents, and flood valves throughout the submarine. Before a submarine dove, the sub’s Diving Officer checked the Christmas Tree to make sure all lights were green, signaling that all hull openings and valves were closed. If any of the lights were red, that meant something was open and it was not safe to dive. If we were on a sub now would it be safe to dive? Although they only made up a small percentage of the US Navy (less than 2%), submarines played an important role during World War II. American submarines sank about 30% of Japan’s warships in the Pacific. But being a submariner was very dangerous, and American submariners suffered the highest casualty rate in the military, losing 52 submarines during the war.