filled in with gravel, although even that did not initially deter many souvenir hunters who measured the position of open hatches and dug down to them to allow the removal of artifacts. The wrecks now lie beneath a car park (parking lot), making them inaccessible. lies in of water west of Key West, Florida. The boat has been visited by divers, but the depth makes this very difficult and the site is considered suitable for only advanced divers. Four other boats lie off the coast of Northern Ireland, where they were sunk during 1946 as part of Operation Deadlight. Both and were found by nautical archaeologist Innes McCartney during his Operation Deadlight expeditions between 2001 and 2003. Both were found to be in remarkably good condition. In April 2018 the wreck of was found north of Skagen in Denmark. Influences The Type XXI design directly influenced advanced post-war submarines, the Greater Underwater Propulsion Power Program (GUPPY) improvements to the United States , , and -class submarines, and the Soviet submarine projects designated , , and by NATO. The Chinese-built Romeo-class submarines, and the subsequent , were based on Soviet blueprints. See also List of Type XXI submarines British R-class submarine Notes References Fitzsimons, Bernard, general editor. The Encyclopedia of 20th Century Weapons and Warfare (London: Phoebus Publishing Company, 1978), Volume 24, p. 2594, "'Whiskey'", and p. 2620, "'Zulu'". External links U-Boot Type XXI in Detail with photos. Type XXI on www.uboataces.com Submarine classes World War II submarines of Germany