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Investigation Confirms WWI Cruiser USS San Diego Was Sunk by U-boat Mine

USS San Diego sinking off Fire Island, New York on July 19,1918. US Navy Painting by Francis Muller

Steaming to New York with a belly full of coal, the armored cruiser USS San Diego (ACR-6) struck a mine deployed by a German U-boat on July 19, 1918. 

The mine exploded along the cruiser’s aft port side, near compartments filled with coal and one of the ship’s four tall stacks. Within 25 minutes, despite initial efforts by the crew to keep the ship steaming ahead as water rushed into its spaces, San Diego continued to list until it sunk in shallow water.

Those findings, presented Tuesday at the American Geophysical Union fall meeting in Washington, D.C, validated earlier determinations including a Navy court of inquiry that San Diego was sunk by a mine deployed by U-156 and not by a torpedo or a saboteur.

The results come from a series of four site visits and dive operations since 2016 conducted by a joint team of Navy historians, academics, researchers and divers with Mobile Diving and Salvage Unit 2 to get definitive answers about what happened to the cruiser. Advanced, hull-mounted sonar along with autonomous and remotely operated vehicles have helped the team develop detailed high-resolution maps of the San Diego’s wreckage site.

A three-dimensional point cloud of survey passes over USS San
Diego the course of a 2017 survey. NHHC Image

The ship’s sinking, to the ocean floor 100 to 110 feet deep some six miles south of Long Island’s Fire Island, was the only major U.S. Navy warship sunk during the war. But the “abandon ship” order by Capt. Harley H. Christy, the ship’s commander, 15 minutes after the initial blast saved all but six men of San Diego‘s 1,183-man crew.

“The question of what sank San Diego has been one that’s been asked for several decades,” Alexis Catsambis, a maritime archaeologist with the Naval Historical and Heritage Command, said during a presentation live-streamed online.

A German U-boat, U-156, was known to have been in the area at the time on a mission to lay mines leading into New York Harbor, according to NHHC. The Navy had first reported its presence publicly a month earlier, on June 26, 1918, and a U-boat was spotted south of Nantucket, Mass., just two days before San Diego was hit.

“We believe U-156 sunk San Diego, and we believed it used a mine to do so,” Catsambis told the audience.

While U-boats also carried torpedoes, “there is no torpedo bubble trail reported by any of the lookouts,” added Ken Nahshon, a research engineer with the Naval Surface Warfare Center, Carderock Division.

The U-boats carried two types of mines, Nahshon said: Torpedo-tube launched and deck-deployed. A close study of the wreckage, computer modeling of water movement during an explosion and crew reports by crew members helped the team determine that

A U-Boat of the same class that may have sunk USS San Diego in 1918.

The 15,138-ton San Diego had arrived in New York City the day before, steaming in from Portsmouth Naval Shipyard, N.H., with the mission to escort ships across the Atlantic. The captain and crew were prepared for the known threats from German U-boats and ships before leaving Portsmouth. On that ill-fated morning, Christy posted 17 sailors as lookouts, “in case a submarine was spotted,” Catsambis said. The ship followed a course, zigging and zagging to avoid being easy prey for the U-boats. Watertight doors were closed.

But an explosion rocked the ship at 11:05 a.m. on a bright morning with clear skies. Despite turning toward the beach with engines at full steam, San Diego sunk into the sea 25 minutes later.

“The captain had done everything right at that point,” Catsambis said, noting, “they were prepared, but tragedy struck.”

“The response to that tragedy is admirable,” he added.

According to NHHC, German Kapitänleutnant Richard Feldt, U-156’s commander, had sent a wireless radio report on Sept. 6, 1918, claiming his crew had sunk San Diego and other ships. In a twist of irony, that same month U-156 met a similar fate when it was sunk by a mine in the North Atlantic. While its logs went down with it, Catsambis noted, it “is credited with the only attack on U.S. mainland,” in an attack against several vessels off Orleans, Mass., just two days after San Diego was sunk.

World War I era cruiser USS San Diego (ACR-6)

The Navy sent airplanes to the site almost immediately after San Diego sunk, and it dispatched salvage divers three times to ensure it wouldn’t become a hazard to navigation. “The dive team verified the site as that of San Diego, lying upside down and resting on its stacks, and revealed areas where the aerial bombs had been dropped on the wreck during the alleged sighting of a German submarine,” according to NHHC.

At the time, the U.S. fleet had little experience with underwater explosions, making the attack on San Diego critical to help understand, prepare and prevent for such events. “These men did everything right in preventing and in responding to a catastrophe off Long Island 100 years ago,” Catsambis said.

The recent dives and investigation have helped detail the mine’s impact on San Diego and how the ship reacted to the explosion. Its thick, armor belt around the hull helped keep San Diego intact, Nahshon said. While crushed at mid-hull section, San Diego remains relatively intact and contained, although its prominent four stacks collapsed and crushed as the cruiser listed and sunk.

The mine struck near the portside steam engine and a void space filled with coal. Water poured into empty spaces, causing the ship to quickly list and sink. Those empty spaces and openings made such coal-fired ships less survivable than oil-fired ships, said Art Trembanis, an associate professor of marine science and policy with the University of Delaware.

The wreckage “is teeming with fish,” Trembanis said, adding “it has become a 100-year-old artificial reef.”

Processed side-scan sonar data collected over USS San Diego during the conduct of Autonomous Underwater Vehicle
operations as part of the 2017 MDSU training mission. NHHC Mission

But danger still lurks at the site. Unlike ships planned to become artificial reefs, San Diego sank full of hazards, from oils and lubricants to live, unexploded ordnance that ordinarily are stripped from ships before they are purposely sunk as ARs. The waters remain heavily navigated and popular for fishing. “There are implications for long-term management and exposure to folks, because it is a very diveable and achievable site,” Nahshon said. The findings will help the Navy manage the site, officials said.

Over the years, time, rough seas, storms and human interaction have taken a toll on the San Diego. Illegal salvage teams and divers had pilfered items including the ship’s propellers, ceramics, personal effects and even ammunition, according to the Navy. The site was placed on the National Register of Historic Place in 1998 and falls under the Sunken Military Craft Act of 2004, which made it illegal to disturb sunken military ships and aircraft without a federal permit. Such wreckage sites remain Navy property and are considered the final resting place for crew members who died.

During the war, the ship, commissioned as the USS California in 1907 and renamed San Diego in 1914, had served as flagship for Rear Adm. William F. Fullam, the Patrol Force U.S. Pacific Fleet commander, according to NHHC. In July 1918, San Diego began a new assignment with the Atlantic Fleet to escort convoys crossing the submarine-infested waters of the North Atlantic. Christy, an 1891 U.S. Naval Academy graduate, continued to serve in the fleet and retired as a vice admiral in 1950, six months before his death.

In July, the joint Navy-private team placed a wreath over the wreckage site to commemorate the 100th anniversary of the sinking of San Diego. Just last month, a collection of 229 artifacts from the San Diego, including an M1892 bugle, padlock and a coffee cup, went on exhibit at the National Museum of the U.S. Navy in Washington, D.C.


  • the_artist_formerly_known_as_m

    “The ship’s sinking, to the ocean floor 100 to 110 feet deep some six miles south of Long Island’s Fire Island, marked the first and only loss of a U.S. Navy ship during World War I.”


    USS San Diego was the only CAPTIAL ship lost during the war. During course of its involvement in WW1 , the US Navy lost four destroyers, several armed transports, a large collier (USS Cyclops) and multiple sub chasers, patrol boats, etc. Not all to enemy action.

    • USNVO

      There are a lot of missing caveats that are not included but “Capital Ship” is not one of them as cruisers, even ones that had been battleships previously, we’re not considered capital ships.

      What I believe they meant to say was the San Diego was the only US Navy ship sunk along the US East Coast by enemy action. Several US Navy ships were lost to enemy action in European Waters and several navy ships sank along the East Coast due to other causes besides enemy action. The San Diego was also the largest USN Warship lost during the war.

      But in any event, a poor choice of words.

      • the_artist_formerly_known_as_m

        Agree, although capital ship is a flexible term. There is no formal definition. Sometimes cruisers are counted (see Germany, WW2). Maybe state major warship?

        USS Cyclops was a commissioned warship and was also larger (greater displacement) than the USS San Diego. But Cyclops was a collier vice combatant, and probably not lost to enemy action.

        • USNVO

          During WWI, capital ships were pretty much universally post-dreadnaught battleships and battlecruisers.

          Cyclops was a collier, and thus by my totally arbitrary definition, an auxiliary and not a warship even if it was armed and commissioned. For that matter, several NOTS transports that were sunk on their return from France were probably larger than the San Diego as well since grt is a measure of enclosed volume, not displacement. But again, those were auxiliaries.

          And although my definition has some internal contradictions, such as an armed transport being an auxiliery while a very similarly armed auxiliary cruiser or raider is a warship, it more has to do with purpose as opposed to design.

          • the_artist_formerly_known_as_m

            I’m not sure I’d even classify the NOTS vessels as “warships”. They were the WW1 equivalent of USNS.

          • USNVO

            I agree, but they were commissioned ships and armed similarly to the auxiliary cruisers the UK used to enforce the blockade against Germany. They were functionally identical, just the purpose they were used for was different.

          • Secundius

            Much like the Boeing B-40, virtually identical to the B-17, but instead of Bombs, a flying Gunship with 18 Heavy Machine Guns. One plane in Six in a Bomber Squadron was assigned in the role of a Flying Gunship. Technically the US Navy didn’t use any “Hilfskreuzer’s” (Auxiliary Cruisers) in WWI, they still had some Thirteen in service during the “Spanish/American War”. Which might have survived to be used in WWI…

    • Secundius

      Keep in mind that the famed “Momsen Lung” wasn’t available before September 1932, and virtually ALL sinking’s of Ship’s and Crew’s and/or Passengers were “Certain Death” at depths beyond 100-feet. The First Practical “Sonar” was available until 1915, and even then you’d have to make a Line Search (i.e. Grid Search) just to find anything…

    • KenofSoCal

      Correct. USS Buena Ventura (ID-1329) was one of those cargo/transports.

  • cgadad

    The USCGC Tampa was lost with all hands from a torpedo attack in 1918. Operating under the Navy at the time, the Tampa was the greatest loss of Naval personnel during the Great War.

  • Secundius

    Mine was most likely a “Hertz” Horn Contact Mine (Type II British designation) with an ~290-pound “Wet Gun Cotton” Warhead…

  • Sam Stewart

    where can I read the complete report see all the scans etc?

  • Chesapeakeguy

    “….in a missile attack against several vessels off Orleans, Mass.,”

    What? Say WHAT? “Missiles”.

    Also, though it was a clear day and many lookouts were posted, it is not impossible for any torpedoes to be undetected. The German sub took credit for sinking the ship. Did it have it under observation? Did it reveal HOW it sank the ship? Not much to go on this article..

    • Secundius

      Sounds like Operation “Teardrop” of 25 October 1944…

  • Eddieksf

    I agree with oddman, the article was not well “proofed,” missile attack ?? You might have also noted in the painting the ship sinking by the bow but the mine exploded off the port quarter. ?? I also dislike the use of phrases which are dubious, like “….only capital ship.” But thats just a cranky cynical 75 yr old… Otherwise, interesting article.