USS S-21 (SS-126) was a first-group (S-1 or "Holland") S-class submarine of the United States Navy in commission from 1921 to 1922 and from 1923 to 1942. In 1928, she made the first gravimetric measurements ever made aboard a U.S. ship at sea. Prior to World War II, she operated in the Atlantic Ocean, Caribbean Sea, and Pacific Ocean, and after the United States entered the war, she operated off Panama. She then served in the Royal Navy as HMS P.553 from 1942 to 1944. Construction and commissioning S-21′s keel was laid down on 19 December 1918 by the Bethlehem Shipbuilding Corporation's Fore River Shipyard at Quincy, Massachusetts. She was launched on 18 August 1920, sponsored by Mrs. Thomas Baxter, and commissioned on 24 August 1921. Service history U.S. Navy 1921–1928 Following operations from New London, Connecticut, S-21 was decommissioned and returned to her builder on 31 March 1922. After she was reacquired by the Navy, S-21 recommissioned at Groton, Connecticut, on 14 September 1923. S-21 then operated off the northeastern coast of the United States. She also visited the Panama Canal, Saint Thomas in the United States Virgin Islands, and Trinidad between January and April 1924; departed New London on 25 November 1924 and visited Hawaii from 27 April to 25 May 1925 before returning to New London in July 1925; performed duty in the Panama Canal area from February through April 1926; visited Kingston, Jamaica, from 20 to 28 March 1927; and operated in the Panama Canal area again from February to April 1928. First U.S. gravity measurements at sea A Royal Navy submarine had made the first gravity measurements at sea in 1926, and the Carnegie Institution for Science subsequently proposed that the United States take such measurements in the Caribbean Sea using a gravimeter designed by Dr. Felix Andries Vening Meinesz. Such measurements required the stability and lack of motion only attainable at sea if taken aboard a submerged submarine. S-21 was selected for the expedition, which was arranged with assistance from the United States National Academy of Sciences, the United States Coast and Geodetic Survey, the United States Naval Observatory — which provided special chronometers necessary for the gravimetric observations — and the United States Navy Hydrographic Office. Meinesz accompanied the expedition, with Elmer B. Collins aboard S-21 to represent the U.S. Navy Hydrographic Office and two Eagle-class patrol craft — USS Eagle 35 and