The loss of the ship became one of the enduring naval mysteries of World War I. For decades afterward, historians and investigators proposed competing explanations, but none could be confirmed with certainty. That long-standing uncertainty persisted until 2018, when an underwater archaeological survey recovered key physical evidence, finally providing new insight into the ship’s destruction and challenging earlier assumptions about what caused the sinking.
Service as the USS California
Laid down and launched on April 28, 1904, the vessel later known as USS San Diego (ACR-6) entered the water several years before her official commissioning into the U.S. Navy. Roughly three years later, she was formally brought into active service and assigned to the 2nd Division of the Pacific Fleet, where her early responsibilities centered on large-scale training operations along the U.S. West Coast.
In March 1912, her operational role broadened significantly when she was reassigned to the Asiatic Station. From there, she operated across East Asia, carrying out missions designed to protect American interests amid regional instability. Her duties during this period included deployments connected to unrest in Nicaragua, patrol activity along Mexico’s coastline, and wider security operations in a volatile geopolitical environment.
By 1914, the cruiser received her final designation as USS San Diego (ACR-6), marking the end of her sequence of name changes prior to the events that ultimately ensured her place in naval history.
Renamed the USS San Diego (ACR-6)
The USS San Diego faced significant mechanical issues early on, including a boiler explosion that led to her being placed on reduced commission in the year following her launch. Despite this setback, she returned to her role as the flagship for the Commander-in-Chief, Pacific Fleet, until February 1917, when she was put into reserve after the United States had not yet entered World War I.
When the U.S. declared war on Germany in April 1917, the San Diego was fully recommissioned as the flagship of the Commander, Patrol Force, Pacific Fleet. Just a few months later, on July 18, the ship was reassigned to the Atlantic Fleet, where she was tasked with escorting convoys through perilous North Atlantic waters, heavily patrolled by German U-boats.
One year later, on July 19, 1918, the ship would face a catastrophic event that highlighted the very real dangers of the sea.
Shaken by an explosion at sea
On July 18, 1918, the USS San Diego left the Portsmouth Naval Shipyard in Maine and headed for New York under the command of Harley H. Christy. The crew stayed vigilant throughout the voyage. Lookouts kept a close eye on the horizon, while fire control teams remained prepared and gun crews stood watch as the vessel traveled in a defensive zigzag pattern.
The following morning, a powerful explosion suddenly tore into the ship’s port side. Crew members immediately tried to control the resulting flooding, but the damage created a serious obstacle. The explosion had distorted the bulkhead, making it impossible to completely close the watertight door separating the engine room from the No. 8 fire room.
Sinking of the USS San Diego (ACR-6)
As water rushed into the ship, Captain Christy ordered full speed ahead, thinking they were under attack by a German U-boat. But the USS San Diego couldn’t accelerate—it was barely able to move. Both of its engines had failed, and the engine rooms were flooding fast.
The ship started to tilt to one side, and within 10 minutes, it was clear it wouldn’t stay afloat. Christy gave the order to lower the lifeboats and abandon ship. Just 28 minutes after the explosion, the San Diego sank beneath the waves, making it the only major U.S. warship lost during World War I.
Out of more than 1,000 sailors on board, six were killed in the disaster.
Survivors were left with no answers
After the sinking, Capt. Christy remained convinced they’d been struck by a torpedo, but there was no evidence that a U-boat had been in the area at the time, and none of the lookouts saw the wake created when a torpedo is fired.
Others speculated it could have been a sea mine, but it’s unlikely one would explode at the stern, instead of the bow of the ship. An official inquiry concluded the sinking was caused by such an explosive, as six contact mines had been located in the vicinity, but the true reason wasn’t that simple.
In 2018, 100 years after the USS San Diego sank, USNI News announced that the cause of the explosion was still inconclusive. Luckily, the American Geophysical Union (AGU) was about ready to hold its annual conference, where a bombshell revelation a century in the making would be dropped.
What really happened to the USS San Diego (ACR-6)?
After two years of research using archival documents, 3D scans and high-tech models, a team of researchers from the Naval History and Heritage Command (NHHC) announced their findings. At the 2018 AGU conference, underwater archaeologist Alexis Catsmabis declared, “We believe that U-156 sunk San Diego.”
Catsmabis explained that the flooding patterns didn’t look like an explosion was set inside the vessel, and the hole ripped into the USS San Diego‘s hull “didn’t look like a torpedo strike,” either. It was concluded that the armored cruiser was struck by a U-boat mine placed by SM U-156.
“Torpedos of the time carried more explosives than mines – and would have shown more immediate damage,” shared marine scientist Arthur Trembanis. The explosion itself wasn’t that powerful, but San Diego was filled to the brim with coal, making her top-heavy enough to easily capsize as she took on water.
“With this project, we had an opportunity to set the story straight,” Catsmabis said in a press release, “and by doing so, honor [the memory of the six crewmen who died] and also validate the fact that the men onboard did everything right in the lead up to the attack as well as in the response.”
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Today, the wreck of San Diego lies upside down off the coast of New York’s Fire Island, some 110 feet below the water’s surface. Since the highest parts are just 66 feet down, the wreck has become a popular scuba diving attraction. It’s also been nicknamed the “Lobster Hotel” for the large community of lobsters that call the armored cruiser home.