A century later, we can now blame a mine from a German U-boat for the lone major US warship sent to the ocean's depths during World War I. The USS San Diego got there fast: The armored cruiser listed and sank within 30 minutes of an explosion off the coast of Long Island on July 19, 1918, leaving six sailors dead, per NPR. But the cause was slow to surface, though researchers at the Naval Surface Warfare Center think they've now solved the puzzle. Dives at the upside-down wreck south of Fire Island—combined with a sonar scan, computer modeling, and crew accounts—revealed that an attack from outside the ship, rather than sabotage or an accidental coal-dust explosion, was the most likely scenario. However, the damage inflicted wasn't so severe as to suggest a torpedo attack, reports Live Science. And none of the ship's 17 lookouts reported a torpedo's bubble trail.
That left much to draw from charts showing German submarine U-156 off Long Island at the time, per NPR. "We believe U-156 sunk San Diego, and we believe it used a mine to do so," maritime archaeologist Alexis Catsambis announced Monday at the annual meeting of the American Geophysical Union in Washington, DC. According to engineer Ken Nahshon, either a mine was laid via U-156's torpedo tube or unloaded from its deck. Researchers say it's comforting to know crew members weren't to blame for the San Diego's sinking, which the ship's captain tried hard to prevent, reports Newsday. Those responsible would ultimately suffer. U-156 sank after hitting a US minefield in the North Atlantic, per Live Science. But first, it committed the only WWI attack on the US mainland when it fired on a Massachusetts town two days after the USS San Diego was sunk. (Read more discoveries stories.)