MIGHTY HISTORY

Navy might know what sank its only major warship lost in WWI

When America joined the Great War, the British Fleet was holding most of the German Navy in the North Sea, meaning that American warships and troop ships rarely faced severe opposition. But one ship did fall prey to an unknown assailant: The USS San Diego, sank off the U.S. East Coast due to a massive explosion from an unknown source.


The USS San Diego in March 1916.

(U.S. Navy)

But the ship is now a fish sanctuary, and researchers looking at the wreck and at historical documents think they've figured out what happened all those years ago.

On July 19, 1918, the armored cruiser was sailing from Portsmouth Naval Yard to New York with a full load of coal in preparation to strike out across the Atlantic. But, as it was coming up the coast, an explosion well beneath the waterline suddenly tore through the ship, hitting so hard that it warped the hull and prevented the closure of a watertight door.

The crew was already positioned throughout the ship in case of trouble, and damage control jumped into action to try to save the ship. Meanwhile, the captain ordered his men to fire the ships massive guns at anything that even looked like a periscope.

USS San Diego sinks in this 1920 painting by Francis Muller.

(Naval History and Heritage Command)

His working theory was that they had been hit by a German torpedo, and he wanted to both kill the bastard who had shot his ship and save the vessel. Unfortunately, he could do neither. The ship sank in 30 minutes into water 110 feet deep, and the crew never spotted the vessel that attacked them.

Six sailors died in the incident. They were Engineman Second Class Thomas E. Davis, Engineman 2nd Class James F. Rochet, Machinist's Mate 2nd Class Frazier O. Thomas, Seaman 2nd Class Paul J. Harris, Machinist's Mate 2nd Class Andrew Munson, and Fireman 1st Class Clyde C. Blaine.

It was a naval mystery for years, but there was a theory competing against the torpedo one: The ship might have struck a mine placed there by a submarine that was long gone when the San Diego arrived.

The proud USS San Diego, also known as Armored Cruiser 6.

(U.S. Navy)

Researchers created a 3-D map of the wreck, and found damage that was most similar to the larger explosive load of a torpedo, but could have been caused by a large mine. And so they turned to naval records handed over by Germany after World War I.

In those records, they found reports from the U-156, a German submarine that did operate on the East Coast that month. But it wasn't concentrating on finding ships to torpedo. She was carrying mines.

"The first thing she did was to lay a string of mines right here, because this was the main convoy route. Most of the convoy routes were coming out of New York City, heading for Europe," Retired Rear Adm. Sam Cox said in July during a ceremony to honor the six sailors lost in the sinking. "We believe those mines were what the San Diego hit."

The mine explosion took place well below the waterline and against relatively thin plating. The mine detonated against a half inch of steel. If it had contacted at the armored band, it would've done paltry damage against the ship's 5-inch thick armor belt.

Because of the limited ships the Central Powers could put to sea in the later years of World War I, the Navy concentrated on protecting and conducting logistics operations rather than chasing elusive fleet action. The Navy delivered more than 2 million soldiers to Europe without losing any soldiers to U-boats.

In World War II, it would be forced to conduct fleet actions while also delivering troops and supplies across the Pacific, Europe, and Africa.