Now, nearly 100 years after it sank, and countless theories as to … New Report Reveals What Happened A team of researchers had been trying to determine whether the … Mystery Blast Sank The USS San Diego in 1918. Sonar scans revealed a detailed view of the USS San Diego, including the hull where the explosion occurred at 11:23 a.m. on July 19, 1918. One of the ships that did not see the war through was the U.S.S. USS San Diego (CA-6), originally the armored cruiser California (ACR-6) USS San Diego (CL-53), a light cruiser commissioned in 1942 in service throughout the Pacific War, and decommissioned 1946 USS San Diego (AFS-6), a combat stores ship in service from 1969 to 1997 USS San Diego (LPD-22), a San Antonio-class amphibious transport dock, launched in 2010 In July 1918, the 15,000-ton armored cruiser USS San Diego sank off Long Island, New York, losing six sailors from a crew of 1,200 after a mysterious explosion struck the vessel.. The USS San Diego sits in 110 feet of water, deep enough that only weak light reaches its portholes. On a summer morning in July 1918, the USS San Diego was steaming off the coast of Long Island, N.Y., when it was rocked by an explosion. USS San Diego may refer to: . The USS San Diego was the only major warship the U.S. lost during World War I. Water rushed into the hull. The damage was extensive and the ship went down within a mere 30 minutes. The USS San Diego is the only U.S. warship to sink during World War I. ... 1918, when an external explosion near the engine room shook the armored cruiser. On July 19, 1918, the USS San Diego had been heading towards New York and was only a few miles offshore when it was suddenly hit by a huge explosion on its port side.