World War I marked the industrialization of warfare, and introduced, for the first time on an international arena, weaponry like war planes, mechanized artillery, tanks, machine guns – and submarines.

But despite having broken out over a century ago, and despite its use of modern technology, some aspects of WWI remain unresolved to this day. In the chaos of war, people, military units and even entire ships can go missing without a trace, and their stories can raise more questions than answers, even after years and years of investigations.

The story of the USS San Diego, which was sunk off the coast of New York in 1918, is one of these mysteries. But unlike many other mysteries, its investigators finally found answers – nearly a century after it was downed.

The USS San Diego was initially named the USS California. The USS California was constructed at the famed Union Iron Works in San Francisco at the turn of the 20th century.

The ship entered service on August 1, 1907, as part of the Pennsylvania-class of cruisers, alongside the USS Pennsylvania, West Virginia, Colorado, Maryland and South Dakota.

These, along with four other Tennessee-class cruisers, were known as the “Big Ten” in the United States Navy, due to their huge size and modern outfitting – but due to technological advancements and new designs, they were soon outclassed by a new type of ship; the Tennessee-class battleship.

By 1914, a new class of war ships had been introduced. In order to free up state names for the new class of ships, the old Pennsylvania-class cruisers changed their names to cities within the states they were originally named for.

Thus, the USS Pennsylvania became the USS Pittsburgh, the USS West Virginia became the Huntington, and the USS California was renamed the USS San Diego.

As one of the United States Navy’s biggest and most advanced vessels at the time, the newly christened USS San Diego had an impressive track record, having served in both the Atlantic and Pacific oceans on various missions, and taking part in countless military activities.

But a mere three years after its name was changed, the armored cruiser would be called to serve in one of the most brutal wars in history – and given a dangerous mission.

On April 6, 1917, the United States joined its allies – Britain, France and Russia – to fight in World War I.

A day later, the USS San Diego was placed in full commission – after a brief, two month period on which she was put on reserve – and began her service in the war as the flagship of the commander of the Pacific Fleet’s Patrol Force, but was soon transferred to the Atlantic Fleet off of the East Coast of the United States.

San Diego’s essential mission was to escort convoys of merchant vessels and other non-combatant ships through the first leg of their dangerous passage through hostile waters, keeping an essential, strategic connection between America and Europe open.

This was a hugely important undertaking, which allowed the United States to stay in touch with its allies during the war, as well as to safely transfer people and much needed weapons and supplies to the European continent that was torn by war.

Operating out of the North American ports of Tompkinsville, New York, and Halifax, Nova Scotia, the USS San Diego escorted ships through hostile, submarine infested waters – often under difficult weather – and could proudly claim that none of her charges were harmed while under her protection.

But despite her distinguished service during the war, the USS San Diego would soon find herself at the bottom of the sea.

In the early morning hours of July 18, 1918, the USS San Diego left the Portsmouth Naval Shipyard and set a course of New York – where the ship’s captain, Harley H. Christy (b. 1870 in Circleville, Ohio), had orders to meet and escort a convoy bound for France.

The ship and crew were serviced and ready, and Captain Christy was confident they would be able to carry out the mission successfully, as they’ve done many times before.

This was a routine mission. The San Diego’s experienced officers and crew had made the journey south to New York countless times, and were eager to get underway with their assignment as soon as possible.

The decks were swabbed, guns were cleaned and the steam ship’s coal supply was full.

Little did they know that this would be the last time the ship ever sailed out into open waters.

A day after they had left port, the crew of the USS San Diego was still making its way south to New York. But then, at precisely 11:05 am, just as the San Diego was passing the Fire Island Lightship, an explosion rocked the port side of the ship.

The San Diego had been hit… very, very close to home.

Could this be the first wave of an attack on US shores?

Something well below the water line had exploded near the port engine room – and damaged the ship in such a way that both the engine room and the adjacent fireroom could not be sealed off from the rest of the ship. The San Diego was taking on water, fast.

But the extent of the damage sustained by the military vessel was not yet clear.

The USS San Diego, being a warship, was equipped to deal with hull breaches.

The ship was built with separate, watertight compartments which could be closed off relatively easily, preventing from water flooding its decks outside of the damaged areas.

In addition to that, the San Diego, like all Pennsylvania-class cruisers, had especially heavy shielding protecting her hull.

So, we wonder, how could just one explosion cause so much damage?

Whatever had hit the ship had managed to breach the hull’s shielding and warp the bulkhead separating two different compartments, which meant that the watertight seal between them had been broken.

Two flooded compartments are much more difficult to contain and deal with than one – and the San Diego was in danger of sinking.

What could have caused so much damage with a single hit?

The explosion had struck the San Diego well below the ship’s water line – which meant that the attack had come from an underwater source. But that didn’t necessarily mean a submarine attack.

While a German U-Boat could definitely have been responsible for this kind of damage, it was also possible the ship had struck an underwater mine.

But then, there was another option, which none of the crew wanted to think about.

Another, much less savory option, was that the ship had been sabotaged – and that one of the sailors had detonated a bomb from inside… or that it had been planted there by a saboteur before the ship even left port.

Could one of the USS San Diego‘s crew have betrayed them?

This simply wasn’t an option anyone was willing to consider.

Upon hearing the explosion, Captain Christy assumed the ship had suffered a torpedo hit. He decided to react with submarine defense maneuvers, which included manning all stations and to search the waters for anything that might look like a periscope.

It also meant getting out of the area as quickly as possible – and so, the captain ordered the ship to steam at full speed ahead on both engines.

It was only then that the extent of the damage began to make itself known.

It didn’t take long after Captain Christy ordered his chief engineer to give him full speed on both engines for him to realize that that would not be possible.

The blast – whether sustained by a torpedo, a mine or a bomb – had done more than flood just one engine compartment and a fireroom.

Because of the extensive flooding, both engines were out of commission – and the ship’s machinery compartments were taking on water as well.

Not long after the Captain was informed of the damage to the engines, things began to look catastrophic.

Because of the continued flooding – and the crew’s inability to close off the damaged area – the ship began to tilt on its side.

Any seasoned sailor will tell you that when a ship begins to tilt, that’s a bad sign.

A very bad sign.

Leaning precariously at a nine-degree angle, water was now no longer just rising the lower decks, but began to rush in through one of the gun ports, flooding the gun deck.

It seemed like the USS San Diego wouldn’t be able to deal with this on her own, and it was then that Captain Christy decided to radio in for help.

The Captain rushed to the radio room, already forming in his head the morbid report he would have to give – but when he got there, the radio operator was flustered.

The blast had knocked out the machinery powering the radio, and the USS San Diego had no way to let Command know they had sustained major damage.

It seemed like the blast had managed, with one fell swoop, to take down all essential systems on the ship.

The Captain would have to make a hard decision.

Captain Christy realized that if the USS San Diego had any chance of surviving the hit she’s taken, he would have to ask for help. Without a working radio, Christy turned to his gunnery officer and, allowing him to handpick a small crew, gave him a boat and sent him to shore, to return with help as quickly as possible.

The San Diego’s crew were now relying on the help of a single boat with a tiny crew – and hoping they, too, wouldn’t be taken down on their way back to port.

A mere ten minutes after she had been hit, Armored Cruiser No. 6, the USS San Diego, began to sink.

Captain Christy ordered the crew to lower the ship’s life rafts and boats, but held off the order to abandon ship until the very last minute. The San Diego was one of the Navy’s best vessels, and he didn’t want to give her up until he was certain the ship would capsize. It seemed, however, that the order was inevitable.

Finally, when Captain Christy was convinced there was no way the ship could be salvaged, he gave the order every ship’s captain dreads giving.

“Abandon ship.”

The crew cried out the order from deck to deck, and sailors, technicians and officers left their posts for the last time aboard what was one of the Navy’s proudest vessels. They boarded the emergency lifeboats and rafts in an orderly, disciplined manner, as Captain Christy stayed aboard to make sure everyone evacuated safely.

Twenty-eight minutes after she had sustained a hit, the USS San Diego succumbed to the cold waters of the Atlantic Ocean.

Out of the ship’s complement of approximately eight hundred and thirty crew members, only six lives were lost. Two of them were killed in the original explosion, one fell overboard, two lost their lives in accidents during evacuation, and one drowned as the ship went down.

While there had been losses in life, none of them, it turned out, were Captain Christy.

The captain was pulled from the water by a crewman by the name of Ferdinando Pocaroba, and placed safely aboard a life raft.

But while the crew had managed to leave the ship safely – they were far from safe. They still needed to be rescued.

While the USS San Diego sunk to the depths, her gunnery officer, with his small boat and crew, had reached shore in the small seaside community of Point O’ Woods, New York.

From there, he was able to contact Navy command, and vessels were immediately dispatched to rescue the San Diego’s crew.

But this was far from being the end of the San Diego’s story.

It had, after all – or at least, so the captain believed – been sunk by an enemy submarine, which was still at large.

Once the crew of the San Diego had been retrieved, it was time for the US Naval Air Service to begin hunting for the submarine responsible for downing the major warship lost by the United States during its involvement in World War I.

The First Yale Unit, based in Bay Shore, Long Island, launched warplanes to scan the seas in the area of the San Diego’s wreck.

Not long after, they spotted something.

Dropping bombs on what the naval pilots thought was a submerged submarine around 100 ft below sea level, for a moment it seemed as if they had caught the submarine responsible for the sinking of one of the Navy’s proudest vessels. But soon after, they realized they were mistaken. They had actually bombed the wreck of the San Diego herself, rather than any enemy vessel – and over a century would pass before the true culprit would be found.

In August 1918, the Naval Court of Inquiry decided an investigation of the sinking of the USS San Diego was in order. The San Diego was the only major warship to be sunk during the war, and understanding the reasons behind its sinking was essential.

This would prove to be, however, much more difficult a task than they had expected.

Captain Christy, an experienced and seasoned naval officer, believed his ship had been sunk by a torpedo. Despite his certainty, there were several issues conflicting with that theory.

First, no submarine had been found in the waters surrounding the ship immediately after its sinking. On top of that, no lookout on the ship had seen the typical, tell-tale wake underwater torpedoes create behind them as they are launched towards a target.

And so, while Christy’s experience was taken into account, a torpedo was ruled unlikely by the investigative committee.

This left only two other options: a mine… or sabotage.

Naval mines have been in use since the early 19th century, and by World War I, their technology had been perfected.

Consisting of an explosive charge surrounded by vials of acid, the mines are triggered when a ship’s hull bumps up against them, crushing the vials which then ignite a battery, which sets off the charge.

Mines can be either free floating – known as “drifting mines” – or anchored to the ocean’s bottom – known as “moored mines”.

Moored mines have the advantage of staying in one place, thereby increasing the likelihood of being struck by a passing ship.

Due to the lack of submarine sightings, as well as there having been found six other mines in the area in which the San Diego has been sunk, a mine seemed like the likeliest cause for the ship’s demise.

Still, something didn’t add up about the mine option, either.

When a ship strikes a mine, she’s most likely to hit it with her bow or forward part of the ship, and not directly on the side.

So could there have been another reason for the San Diego’s sinking?

If the ship hadn’t been struck by a torpedo, and hadn’t been hit by a mine – could she have been sabotaged?

In 1999, a theory was advanced that the famed German spy and saboteur, Kurt Jahnke (1882-1945), had planted explosives aboard the ship, causing an explosion in a strategically chosen part of the ship.

This theory wasn’t without merit – Jahnke was based in San Francisco, as was the USS San Diego before she left for the East Coast – but the Naval Historical Center contested the theory.

In August 1918, the investigation concluded that the cause of the San Diego‘s sinking was due to a naval mine.

Still, many of the involved parties remained unconvinced, and the case of the San Diego remained an open – albeit dusty – one, for nearly a century.

In July 2018, the United States Navy News Service reiterated the reason behind the San Diego’s sinking was still unknown – but that would change, later that year.

Over the years, forensic – as well as underwater – technology had greatly advanced.

The case of the USS San Diego continued to intrigue and bamboozle investigators, who were eager to understand how the only major downed United States warship in World War I had been sunk.

In December 2018, at the annual meeting of the American Geophysical Union, this question would finally be answered.

At the 2018 annual American Geophysical Union meeting, a young underwater archeologist by the name of Alexis Catsambis, working with the Navy, stated that “We believe that U-156 sunk San Diego”. Flooding patterns in the ship “weren’t consistent with an explosion set inside the vessel,” thereby removing any chance of sabotage, while the hole “didn’t look like a torpedo strike.”

So what was U-156?

In recent years, records of the German army during WWI were released to the public. In them, there is documentation of a submarine named U-156, which had operated along the south shore of Long Island.

U-156 was a mine-laying vessel, and it is believed, after the extensive underwater archaeological work done by Alexis Catsambis and his colleagues, that it was mines deposited by the submarine were responsible for the sinking of the USS San Diego.

Finally, the mystery had been resolved.

But what’s happened to the San Diego since?

Today, the USS San Diego lies under 110 ft (34 meters) of water, off the coast of Fire Island.

These are considered comfortable diving depths, and thus, the San Diego has become one of the most popular shipwrecks in the United States for SCUBA diving.

Unfortunately, because the wreck lies upside down, it is not a safe diving location for inexperienced divers, and over the years it has claimed more lives in diving accidents than it had while sinking.

Still, the site remains extremely popular, and is nicknamed “Lobster Hotel” by local divers thanks to the abundance of lobsters that have made it their home.

The USS San Diego‘s story is a fascinating one.

It has stood as an unanswered mystery for nearly a century, baffling countless researchers and military historians, sparking up theories, controversy and interest in all who hear it.

It’s amazing to think that after a hundred years of questions, we are finally able to provide answers to one of the most important wrecks in WWI US Naval history – but more than that, it makes us wonder what other unanswered questions lie beneath the sea, waiting to be uncovered… and answered.