U-20 was a German submarine, which was built on December 18, 1912, and joined the Imperial German Navy on August 5, 1913. During the First World War she participated in the German blockade of the British Isles governed by lieutenant commander Walter Schwieger. On May 7, 1915, 11 nautical miles north of Ireland and within the exclusion zone, U-20 torpedoed the Lusitania ocean liner, which was requisitioned by the British, while she was approaching Europe coming from New York. The ship sank within 18 minutes drifting 1,198 people into the watery grave.

The sinking caused a wave of anger in the USA since 128 of the victims were American citizens, whilst at the same time the British insisted that Lusitania did not carry war material. It is certainly noteworthy that in 1982 the British Ministry of Foreign Affairs admitted that a large quantity of explosives is located in the shipwreck, which constitutes it dangerous for the divers who attempt to visit it.

The protests expressed at that time had as a result the explicit assurance on behalf of the German emperor (Kaiser) that he has given orders henceforth that all the passenger ships should not to be attacked. Yet, on September 4, 1915, lieutenant commander Schwieger disobeyed the orders, and sank yet another passenger ship, namely Hesperian, 85 nautical miles southwest of Ireland. Due to this fact, once the U-20 returned to her basis, the governor was arrested and imprisoned. 2 years later, Schwieger died; he was acquitted of the charges and was decorated for the sinking of ships totaling 190,000 tons. As for U-20, her end was equally inglorious. On November 4, 1916, U-20 ran aground on a Danish coast and was abandoned by her crew after they first used explosives to render her operationally useless.