Navy: U.S.S. San Diego Was Sunk By German Mine in 1918

A century-old mystery off the Long Island shore has been solved by the U.S. Navy.

The USS San Diego, an armored cruiser scheduled in the summer of 1918 to escort a convoy across the Atlantic Ocean to assist Allies in the fighting of World War I, was sunk after an explosion just off the shore of Fire Island, where it remains to this day, a prime wreck diving spot.

It was the only major U.S. warship sunk in World War I.

The explosion, on the port side near the engine room below the waterline, was initially believed to be caused by a German torpedo, and the ship’s captain, Captain Harley H. Christy, immediately ordered all guns to open fire on anything resembling a periscope. 

A Naval Court of Inquiry the following month found no evidence of U-boats or torpedoes in the area, but did find several mines, leading to the conclusion that a German mine-laying submarine had earlier laid mines there.

A groundbreaking survey in the fall of 2017, led by Naval History and Heritage Command and the University of Delaware’s College of Earth, Ocean and Environment, has borne out that conclusion.

The results were released this year as part of the commemoration of the centennial of the end of World War I.

Dr. Alexis Catsambis of the Naval History and Heritage Command’s Underwater Archeology Branch announced Dec. 11 that research team believed the explosion’s cause was one of the two types of mines laid by the German submarine U-156.

That German submarine wreaked havoc along the Eastern Seaboard throughout 1918, sinking barges and a tugboat in the Massachusetts town of Orleans and sinking 21 fishing bots in the Gulf of Maine before it was lost. The U-boat is believed to have been destroyed after striking a mine in an Allied minefield between the United Kingdom and Norway. 

“The legacy of the incident is that six men lost their lives on July 18, 1918,” said Dr. Catsambis of the sinking of the San Diego. “With this project we had an opportunity to set the story straight and by doing so, honor their memory 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. The fact that we lost six men out of upwards of 1100 is a testament to how well they responded to the attack.”

Ken Nahshon, Ph.D., of the Naval Surface Warfare Center Carderock Division and Arthur Trembanis, Ph.D. from the University of Delaware in Newark, Del., also shared their findings, in the Dec. 11 panel discussion at the American Geophysical Union’s Fall Meeting in Washington, D.C.

U.S. Naval Academy Midshipman 1st Class Nolan Brandon launches a remotely operated vehicle during a survey of the wreck on Sept. 13. | U.S. Navy photo by George Schwarz, Ph.D.
U.S. Naval Academy Midshipman 1st Class Nolan Brandon launches a remotely operated vehicle during a survey of the wreck on Sept. 13. | U.S. Navy photo by George Schwarz, Ph.D.

The scientists detailed how each of their teams used historical analysis, archaeological research, site investigation, and impact and flood modeling to eliminate other possibilities that might have caused San Diego’s sinking, including sabotage, accident, or enemy torpedo.

The team used underwater robotics and remotely deployed instruments, including an autonomous underwater vehicle, to collect high resolution 3D images of the site to support their conclusion.

“The format of the 3D modeling data makes analysis readily comparable,” said Dr. Nahshon. “Before we started this, I wasn’t familiar with the ability to do this underwater. Above the water we do it all the time, but below water, collecting 3D data is a challenge. I’ve learned that the sheer amount of expertise that’s needed to interpret it is a credit to the advances of technology in sea floor mapping.”

“The collection of archeological and hydrographic data establishes a baseline informing site formation processes and management of USS San Diego,” said Dr. Catsambis. “Lessons learned here are applicable to other U.S. Navy sunken military craft. This endeavor also provided real-world training opportunities for U.S. Navy divers, archaeologists, historians, modelers, naval engineers and graduate students.”

Beth Young

Beth Young has been covering the East End since the 1990s. In her spare time, she runs around the block, tinkers with bicycles, tries not to drown in the Peconic Bay and hopes to grow the perfect tomato. You can send her a message at editor@eastendbeacon.com

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