USS Eagle 58 — serving as tenders. The expedition set out from Hampton Roads, Virginia, on 4 October 1928 and followed a route that took it to Key West, Florida; Galveston, Texas; Guantanamo Bay, Cuba; the United States Virgin Islands; and Puerto Rico, with a return to Hampton Roads on 30 November 1928. The first of 49 gravity measurement stations S-21 occupied was at . The gravity measurements were accompanied by a sonic depth sounding with results published on a chart produced by the U.S. Navy Hydrographic Office. The gravity measurements were used in determining the shape of the Earth, and of particular interest were significant negative gravity anomalies the expedition detected. The results of the expedition, particularly the negative anomalies, created interest in a second U.S. gravimetric expedition at sea, resulting in the Navy-Princeton gravity expedition to the West Indies in 1932 using the submarine and a third measurement effort in 1936–1937 — the Gravimetric Survey Expedition — using the submarine . 1928–1941 After returning from her gravimetric expedition, S-21 resumed operations from New London along the northeast coast of the United States. She also served in the Panama Canal area from March to April 1929 and again from January through February 1930. Departing New London on 22 October 1930, S-21 transited via the Panama Canal and California to Pearl Harbor, Hawaii, arriving there on 7 December 1930. From 1931 into 1938, S-21 operated from Pearl Harbor, with the period 18 November 1932 to 24 January 1934 spent in reserve. Departing Pearl Harbor on 15 October 1938, S-21 transited via California and the Panama Canal to Philadelphia, Pennsylvania, arriving on 11 December 1938. Following overhaul she arrived at New London on 25 March 1939. She remained at New London with a partial crew from 1 June 1939 until 1 September 1940, when she returned to full commission with a full crew. World War II The United States entered World War II with the Japanese attack on Pearl Harbor on 7 December 1941. On 9 December 1941, S-21 got underway from New London bound for the Panama Canal Zone. Arriving on 19 December 1941, she conducted defensive war patrols in the Pacific Ocean approaches to the Panama Canal through May 1942, although her second such patrol, scheduled for 24 January to 7 February 1942, was cancelled to allow her to participate in search and rescue operations for the submarine ,