was a Japanese Type B1 submarine which saw service during World War II in the Imperial Japanese Navy. She displaced 1,950 tons and had a speed of . I-21 was the most successful Japanese submarine to operate in Australian waters, participating in the attack on Sydney Harbour in 1942 and sinking 44,000 tons of Allied shipping during her two deployments off the east coast of Australia. Service history The submarine was laid down on 7 January 1939 at the Kawasaki shipyard, Kobe, and launched on 24 February 1940. On 15 July 1941 she was completed, commissioned and assigned to Submarine Squadron 1's Submarine Division 3 in the Sixth Fleet. I-21 was based in the Yokosuka Naval District. On 31 October 1941 Commander Matsumura Kanji was assigned as Commanding Officer, and on 10 November he attended a meeting of submarine commanders aboard the light cruiser , convened by Vice Admiral Mitsumi Shimizu, to be briefed on the planned attack on Pearl Harbor. Attack on Pearl Harbor I-21 departed Yokosuka on 19 November and sailed to the rendezvous at Hitokappu Bay, Etorofu, arriving on the 22nd, and departing on the 26th for the Hawaiian Islands, acting as a lookout ahead of the Carrier Striking Force. On 2 December 1941 the coded signal "Climb Mount Niitaka" was received, signifying that hostilities would commence on 8 December (Japan time). On 7 December 1941 I-21 was assigned to patrol north of Oahu, Hawaii. On 9 December I-6 reported sighting a and two cruisers. I-21 and the rest of SubRon 1 boats, were ordered to pursue and sink her. However I-21s pursuit was delayed by diesel engine breakdowns and electrical problems. She was also spotted by several Douglas SBD Dauntless dive-bombers and forced to dive each time. Finally, on 14 December, the chase was abandoned and I-21 and the other submarines were ordered to the West Coast of the United States to attack American shipping. I-21 was assigned to patrol off Point Arguello, California. Sinking of SS Montebello On 23 December 1941, I-21 sighted the Union Oil Company's oil tanker . The vessel, built in 1921, was en route from Port San Luis, California, to Vancouver, British Columbia. At 05:30, I-21 fired two torpedoes at a range of . One was a dud, but the other struck forward in the pump room and dry storage cargo hold. The 38-man crew abandoned the tanker in four lifeboats,