USS California (ACR-6)

Early on 18 July 1918, San Diego left the Portsmouth Naval Shipyard bound for New York where she was to meet and escort a convoy bound for France. Her captain — Harley H. Christy — ordered a zigzag course at a speed of 15 kn (28 km/h; 17 mph). Visibility was reported as being from 6–8 mi (9.7–12.9 km). In his report to a Board of Inquiry on the cruiser's loss, Christy stated that all lookouts, gun watches, and fire control parties were at their appointed stations and on full alert, and that all necessary orders to safeguard the watertight integrity of the ship in dangerous waters had been given and were being carried out.^[11]

At 11:05 the next day, 19 July, San Diego was steaming northeast of the Fire Island Lightship when an explosion occurred on the cruiser's port side adjacent to the port engine room and well below the waterline. The bulkhead at the site of the explosion was warped so that the watertight door between the engine room and No. 8 fireroom could not be shut, and both compartments immediately flooded. Captain Christy assumed that the ship had been torpedoed and immediately sounded submarine defense quarters and ordered all guns to open fire on anything resembling a periscope. He called for full speed ahead on both engines and hard right rudder, but was told that both engines were out of commission and that the machinery compartments were rapidly flooding. The ship had taken on a 9° list and water began pouring in through one of the 6-inch (152 mm) gun ports, flooding the gun deck.^[11] As water poured into the gun deck it also entered coal chutes and air ducts, further increasing list.^[12]

Informed that the ship's radio was not working, Christy dispatched the gunnery officer to the mainland with a boat crew to summon rescue vessels.

About 10 minutes after the explosion, the cruiser began to sink. Orders were given to lower the life rafts and boats. Captain Christy held off giving the order to abandon ship until he was certain that San Diego was going to capsize, when the crew abandoned the vessel in a disciplined and orderly manner. Christy was rescued by a crewman named Ferdinando Pocaroba.^[13] The vessel had sunk in 28 minutes with the loss of six lives, the only major warship lost by the United States after its involvement in World War I. Of the six fatalities, two men were killed instantly when the explosion occurred, a crewman who had been oiling the port propeller shaft was never seen again, a man was killed by one of the smokestacks breaking loose as the ship capsized, one was killed when a life raft fell on his head, and the sixth was trapped inside the crow's nest and drowned.^[14]

Meanwhile, the gunnery officer had reached shore at Point O' Woods, New York after a two-hour trip, and vessels were at once sent to the scene.

The Navy Department was informed that a German minelaying submarine was operating off the east coast of the US and the US Naval Air Service was put on alert. Aircraft of the First Yale Unit, based at Bay Shore, Long Island, attacked what they thought was a submerged submarine lying on the seabed in around 100 ft (30 m) and dropped several bombs; it turned out to be San Diego.

Captain Christy was of the opinion, following the sinking, that San Diego had been sunk by a torpedo. However, there was no evidence of a U-boat in the area at the time, and no wake of a torpedo was seen by the lookouts. While it was reported that five or six mines had been found in the area, the idea that she had struck a mine was also considered unlikely as it was thought that a mine would have been more likely to detonate at the bow or the forward part of the ship.^[15] It was subsequently reported that experienced merchant officers believed that a mine was the probable cause, due to the violence of the explosion and the rapidity with which the ship sank.^[16] In August 1918, the Naval Court of Inquiry appointed to investigate the loss of the cruiser concluded that San Diego had been sunk by a mine, mentioning that six contact mines had been located by naval forces in the vicinity of the spot where she had sunk.^[17]

In 1999, a theory was advanced that a German spy Kurt Jahnke had planted explosives aboard causing the sinking.^[7][18] The claim was contested by the Naval Historical Center.^[7]

In July 2018, USNI News reiterated that the cause of the sinking of San Diego was still unknown.^[19][20] However, the German submarine, U-156 had earlier laid a number of mines along the south shore of Long Island,^[21] and the sinking of San Diego was attributed to her.^[14][22] Naval records recovered in Germany after the war had shown that U-156 had been operating off the coast of New York, laying mines.^[23]

In December 2018, at the annual meeting of the American Geophysical Union, Alexis Catsambis, an underwater archaeologist with the Navy, stated "We believe that U-156 sunk San Diego".^[23][24] Flooding patterns studied "weren't consistent with an explosion set inside the vessel", while the hole "didn't look like a torpedo strike." "Torpedoes of the time carried more explosives than mines – and would have shown more immediate damage," stated University of Delaware marine scientist Arthur Trembanis, who took part in the latest wreck study. Mines were anchored at optimal depths to tear open warships, according to Ken Nahshon, another researcher. The mine in question hit an "unguarded lower part of the ship, where the hull was only about a half-inch thick", he argued.^[25]