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How USS San Diego Sank is Still an Open Question 100 Years Later

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

Mine or torpedo? For a hundred years, the cause of the explosion that sank USS San Diego (ACR-6) has been a mystery. But now researchers are on an expedition to determine what exactly caused a Navy armored cruiser to sink 100 years ago within eyesight of New York City as the U.S. was entering World War I.

On July 19, 1918, San Diego was returning to New York from Portsmouth N.H., in preparation to escort a convey to Europe. At about 11:05 a.m., while steaming just south of Long Island, an explosion ripped open the midship section, causing the ship to quickly list 17 degrees, flooding the hull and ultimately sinking 28 minutes later, according to the Naval History and Heritage Command (NHHC).

“For the six guys who lost their lives on San Diego, they were nearing New York City, it was 11 o’clock in the morning, they were probably thinking about liberty and it all changed in an instant,” Paul Taylor, a NHHC spokesman told USNI News.

Now, on the hundredth anniversary of San Diego’s sinking, the Navy held a wreath laying ceremony at the surface above where the ship sank and sent a special dive team down to the sea floor to try determining exactly what caused the sinking.

For much of the past century, Navy officials, historians and scuba divers visiting the wreck thought a torpedo caused the explosion, but recent surveys of the wreck, using high-tech scanning and sonar equipment, suggest the blast may have been caused by a sea mine instead, Taylor said.

A German submarine, U-156, is believed responsible for sinking San Diego – the only major U.S. Navy ship lost in the first world war, according to the NHHC. U-156 was sent to lay a string of mines outside of New York Harbor, but details of the submarine’s activities are not known because U-156 never returned to port, according to the NHHC.

A survey of the site last September provided good sonar data, but the visual data was not conclusive to determine whether a mine or torpedo caused the sinking, Taylor said. Researchers wanted another try at surveying the site when weather conditions would be considered favorable for a survey.

The anniversary of San Diego’s sinking happens to align with a time when the Navy’s Mobile Diving and Salvage Unit Two was due for a training exercise and the Military Sealift Command rescue and salvage ship USNS Grasp (T- ARS-51) was available, Taylor said. Divers from the Salvage Unit Two are visiting the wreck to provide visual details of the site that were not available during recent surveys.

What sets San Diego apart from other shipwrecks is the ship’s location is relatively easy to access — it’s been a favorite spot for recreational scuba divers for years – and it offers researchers a rare peek at what happens to a hull after being sunk by a mine, Taylor said.

“It’s unusual to have such good access to a ship that was sunk [like that],” Taylor said.

The location also provides a challenge for the NHHC, Taylor added. Over the years, contrary to federal law, divers have scoured the wreck for souvenirs. Both propellers have been removed but lost at sea, and at least six recreational divers have died trying to illegally recover artifacts from the wreck, according to the NHHC.

“The wreck appears to be in pretty good condition,” Taylor said. “It’s is starting to show some deterioration midsection where the explosion occurred, and the hull is starting to cave in.”

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

The wreck is still Navy property, and the site is considered the final resting place for six sailors who died – their final resting place marked by the turtled hull on the sea floor about 100 feet below the sea surface, southeast of Fire Island N.Y.

The Navy can potentially use information about how the ship sank and its condition a century later in ship design, Taylor said. Usually, ships sunk by mines are not easily accessible, because of their location in deep waters or far from shore. Or, as happens frequently, after a conflict, the local authorities concentrated on removing shipwrecks consider navigation hazards without considering any lessons to be learned from how the ships sank, Taylor said.

Aside from the lessons learned, Taylor said it’s important for the Navy to keep an eye on these sites, which are considered hallowed ground just as is the case with military cemeteries. For sailors today, visiting sites of wrecks sends a powerful message, Taylor said.

“It’s important for them to know their service and their sacrifice will be remembered,” Taylor said.

  • proudrino

    ‘what exactly caused a Navy armored cruiser to sink 100 years ago within eyesight of New York City as the U.S. was entering World War I’

    Somebody at USNI or the NHHC command needs to get a better understanding of history. The United States entered WWI on 6 April 1917. The units of Destroyer Division 8, were the first American destroyers to arrive at Queenstown, Ireland on 4 May 1917. Armistice was declared on 11 November 1918. So…… to characterize the time of the San Diego’s sinking on 19 July 1918 as when ‘the US was entering WWI’ is not accurate.

    • DaSaint

      I thought the same thing. More like the final months of the war as opposed to ‘as the U.S. was entering World War I’.

    • Alfred E. Packer

      Kind of casts some doubt on the credibility of the article (and author).

      • TomD

        It would seem that this line was thrown in to possibly exclude any charge of complacency on the part of the officers and crew – not that I know of any such charge, just sayin’.

  • William Blankinship

    Let is be there in peace. Nothing can be learned from it about ship design. This is just another way to waste money.

    • Fred Gould

      From a nondestructive examination view point the answer is yes. Every wreck dive has to potential to improve our understanding.

  • Ruckweiler

    RIP, Shipmates.

  • Bill

    My maternal grandfather served on a sister ACR in the Great War. RIP.

  • Ed L

    Mines are indiscriminate when it comes to picking their targets. I like that line about the six sailors waiting liberty. Many people who are relatives in the Navy or where in the navy Have heard mention of liberty cards. Liberty wasn’t always a given back them. It’s well known but those in the great White Fleet cruise roughly 20% of the enlisted men on the ships got liberty. The rest didn’t. Those that did get liberty were the most well behaved honorable and cleanest of the sailors on the ships

    • proudrino

      You’ve hit on a pet peeve of mine. Many historians focus on what past generations were feeling. It is a way to make historical events seem more human to contemporary readers but it is often at the expense of historical accuracy. Society a century ago simply did not operate the way it does now. The life of the enlisted crew of a Navy ship in 1918 was far different than they are today. The movie The Sand Pebbles gives a slight glimpse into some of the differences.

      Those who claim to know what individuals were feeling in the past (with no documented evidence like letters or diaries) are more social scientists than historians. The crew of the USS San Diego was operating in wartime and preparing to perform convoy duty. They were probably not focused on liberty. NHHC should apply more academic rigor to their press releases.

      • Ed L

        I still remember the signs (Sailors not allowed) in the windows of many restaurants and some stores in Norfolk. I remember One time in 72 a couple of us sailors were walking in downtown Norfolk, checking out the fancy restaurants and a police Officer on beat to us sailors to stop loitering and move along

  • John B. Morgen

    A German mine makes more sense than the cruiser being sunk by a German torpedo.

  • R’ Yitzchak M

    Under the declassified British Navy data one of the best kept secrets that eventually made a huge paradigm shift on naval warfare is a success of German mines during the WW2 OF ALL TONAGE SUNK 78% were due to the mines and not the subs. Thanks G-d Germans did not got hang of it..

  • El_Sid

    Ironic that the class of submarine was originally designed to allow Germany to trade with the US despite the Anglo-French blockade – the one pictured in London above was technically a merchantman for most of WWI.

    Having designed an unarmed sub with the range to travel from Germany to the US for trade, it wasn’t such a stretch to arm them and build new armed ones that could make the same journey for different purposes…

  • Secundius

    My guess is Where the Coal Bunkers were in relationship to the Coal Fired Steam Boilers were. If the Coal Bunkers were “Uninsulated” and the Temperature of the Boiler Room exceeded +150F, which it probably did. Coal can “Bleed” into to Combustible Gases components like Hydrogen…