1. Germany turned out over 1000 U-boats during the Second World War. Most of these were built between 1941 and 1945.
2. Hamburg turned out a third of Germany's U-boats. The city was heavily targetted by Allied bombers.
3. Seventy percent of German U-boats were lost, missing in action or scuttled at sea. The Allies' organized convoys and improved submarine tactical countermeasures caused this high percentage.
4. The first ship to be sunk by a U-boat during World War II was the HMS Indomitable.
5. U-boat stands for the German word Unterseeboot. Winston Churchill declared: "Enemy submarines are to be called U-boats. The term submarine is to be reserved for Allied underwater vessels. U-boats are those dastardly villains who sink our ships while submarines are those gallant and noble craft which sink theirs."
6. Karl Donitz, "The Father of the U-boat Weapon", was instrumental in organizing Operation Hannibal, evacuating hundreds of thousands of East Prussians across the Baltic Sea fleeing the Red Army.
7. Most U-boats were sunk not by ships but by aircraft.
8. British Commandos hijacked a German U-boat which had an enigma ciphering machine on board. This gave them vital information about the Nazis which helped them invade the Normandy beaches on June 6, 1944. Without this information, D-Day likely wouldn't have taken place until 1945.
9. U-boats were under the jurisdiction of the Kriegmarine, the German Navy.
10. Boom nets, mines and waves were just some of the obstacles faced by U-boats.
German U-boat later sunk in Churchill River, Labrador courtesy http://padiprosnorthamericacaribbean.com/2012/10/12/sunken-wwii-nazi-u-boats-found-in-canada/.