This Military Mystery Below The Ocean Was Finally Solved After 100 Years

World War I brought around the deadly introduction of mechanized warfare onto the battlefield, with machine guns, planes, ships, submarines, and more all being used at once for the first time in human history. Despite our technological advancements made since the Great War, some mysteries from the war still remain unsolved. Countless aircraft, ships, and military units disappeared without a trace, and it’s taken decades to discover what happened. This is the case of the USS San Diego, an armored cruiser that was mysteriously sunk off the coast of New York in 1918. Finally, after years of research, investigators finally have an answer.

It Was One Of The “Big Ten”

One of the big ten
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Constructed at San Francisco’s Union Iron Works in the early 1900s, the USS San Diego originally went by the USS California. Commissioned on August 1, 1907, the ship was a part of what’s known as the Pennsylvania-class of cruisers along with the USS Pennsylvania, West Virginia, Colorado, Maryland, and South Dakota.

They were a part of the United States Navy “Big Ten” along with four other Tennessee-class cruisers that earned their names due to their behemoth size and modern designs. However, the Tennessee-class ships would eventually surpass the others.

The Name Had To be Changed

Men building a ship
Clem Albers/San Francisco Chronicle via Getty Images
Clem Albers/San Francisco Chronicle via Getty Images

In 1914, a new class of warships began to be developed, and states’ names could no longer be attached to ships, so the name USS California had to be changed. Much like the rest of the Pennsylvania-class cruisers, it was named after the state that it was initially from.

So, the USS California became the USS San Diego, the USS Pennsylvania became the USS Pittsburgh, and the USS West Virginia became the USS Huntington. Luckily, this wasn’t a big deal.

It Was A Well-Known Ship

Men getting off ship
Bettmann/Getty Images
Bettmann/Getty Images

The USS San Diego was one of the United States Navy’s largest and most advanced ships of its time. It was also put to good use, participating in military missions all over in the world in both the Atlantic and Pacific Oceans.

However, just three years after receiving its final name, the USS San Diego was put to its biggest test. It set off to go fight in the deadliest war that the world had ever see, World War I.

America Enters The War

President Wilson with Congress
Topical Press Agency/Getty Images
Topical Press Agency/Getty Images

On April 6, 1917, the United States officially declared war on Germany, agreeing to fight alongside Britain, France, and Russia in World War I. The United States troops were shipped onto the shores of Europe, with many American ships ready to fight, including the USS San Diego.

After two months of being on reserve, the ship began operation as the commander of the Pacific Fleet’s Patrol Force, before being moved to the Atlantic Fleet off of the East Coast.

Its Mission Was Clear

Massive WWI ship
Universal History Archive/Universal Images Group via Getty Images
Universal History Archive/Universal Images Group via Getty Images

During the course of the war, the USS San Diego had clear instructions on what its purpose was. Its mission was to keep the strategic connection between the United States and Europe by escorting merchant vessels and other non-combatant ships through the first part of the dangerous passage through hostile waters.

In doing this, the USS San Diego made it, so the United States never lost connection with their allies during the war, while also making sure people, weapons, and other supplies were delivered to soldiers in Europe.

It Had A Dangerous Job

Warship next to small boat
Paul Thompson/FPG/Getty Images
Paul Thompson/FPG/Getty Images

The USS San Diego was based in the North American ports of Tompkinsville, New York, as well as Halifax, Nova Scotia. However, its primary mission proved to be a difficult one.

The ship was operating in submarine-infested waters while simultaneously battling harsh seas, yet nothing proved to be a problem. This is why it was so hard for people to believe that a ship of this nature would soon end up on the seafloor.

Heading To New York

Men looking through scopes
Jacques Boyer/Roger Viollet via Getty Images
Jacques Boyer/Roger Viollet via Getty Images

In the early morning of July 18, 1918, the USS San Diego left Portsmouth Naval Shipyard and set a course to New York. There, the ship’s captain, Harley H. Christy, had instructions to meet and safely escort a cargo ship that was on its way to France.

Both the ship and her crew were more than ready for the mission with Captain Christy confident that they would successfully get the job done. After all, they had done this many times before.

Nothing Unusual About The Mission

Men repairing ship
Hulton Archive/Getty Images
Hulton Archive/Getty Images

At this point, the men aboard the USS San Diego were skilled sailors and had completed plenty of missions like this before.

To them, this was just another mission like any other. The men were ready to finish this assignment and get onto the next. In preparation, the crew swabbed the decks, checked the ship, cleaned the guns, and headed towards New York.

An Explosion Changed Everything

Explosion in water
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ullstein bild/ullstein bild via Getty Images

Just a day after leaving the port, the crew of the USS San Diego was positive that they were going to have a successful mission. Yet, as they were making their way south into New York City, something unbelievable happened as they were passing the Fire Island Lightship.

At 11:05 on July 19, 1918, a massive explosion occurred on the port side of the USS San Diego, endangering the lives of those on board. They then assumed their ship had been hit.

It Severely Damaged The Ship

Illustration of a ship
Universal History Archive/Universal Images Group via Getty Images
Universal History Archive/Universal Images Group via Getty Images

The entire crew of the USS San Diego was taken by surprise, and they knew that it was only the beginning of their troubles. Both the engine room and the fire room had taken on serious damage, as the explosion had occurred below the waterline.

As if things weren’t bad enough, the part of the ship that hadn’t experienced much damage couldn’t be sealed off. On top of that, nobody could tell how badly the ship had actually been harmed.

The Ship Was Built For Such Situations

Ship in the water
Library of Congress
Library of Congress

Like the other warships of its kind, the USS San Diego was built to handle incidents like this. In particular, it was built to deal with hull breaches, being equipped with separate, water-resistant compartments that the crew could close off to prevent water from flooding the decks.

Furthermore, the ship was even more resilient because it had shielding protecting its hull. This is one of the key reasons that experts wonder how such an explosion managed to compromise such a mighty vessel.

There Was Little That Could Be Done

Exterior of a ship
Library of Congress
Library of Congress

Unfortunately, the explosion was so intense that it breached the hull’s shielding, and everything started to go south. The bulkhead that separated the ship’s compartments had been terribly warped, and the watertight seal in the middle of them had been compromised.

Although dealing with one compartment flooded with water could have been manageable, there were two, which made all the difference. At this point, the ship was at risk of sinking into the sea.

Could It Have Been An Underwater Attack?

Underwater mines
Roger Viollet via Getty Images
Roger Viollet via Getty Images

Although the USS San Diego had a history of operating in areas with potential submarines, it seemed a little too far-fetched for there to be any that close to New York City.

The explosion had occurred below the ship’s waterline, but that didn’t necessarily mean that it came from a submarine. It wasn’t out of the question that a German U-Boat could have been involved, but it was also possible that they had hit an underwater mine.

Possible Betrayal

Explosion in the water
Hulton Archive/Getty Images
Hulton Archive/Getty Images

Even though none of the crew wanted to think and let alone talk about it, there could be a possibility that the attack came from the inside. A bomb could have been planted long before the USS San Diego left port to head to New York and detonated by someone once the ship was right on top of it, as they knew the course that it would take.

Although it was something nobody wanted to admit, the thought that there could be a traitor on board couldn’t be ruled out.

Preparing For A Fight

Man with artillery gun
Bettmann/Getty Images
Bettmann/Getty Images

Not long after the ship was hit, Captain Christy quickly came to the conclusion that the USS San Diego had been torpedoed. Thinking quickly, he gave orders for submarine defense orders, had all stations manned, and ordered them to open fire if they say anything that looked like a periscope.

Nevertheless, it was crucial that they get out of the danger zone as soon as possible. Christy then called for full speed ahead.

The Engines Were Busted

Ship with smoke
Paul Thompson/FPG/Getty Images
Paul Thompson/FPG/Getty Images

While the crew had orders to get the ship out of the hostile waters as quickly as possible, it became apparent that this wasn’t possible. Although nobody was confident where the explosion had come from yet, what was clear is that it didn’t just flood the fireroom and one engine compartment.

Tragically, at this point, all of the engines were out of commission. This was due to heavy flooding, and all the compartments had filled with water.

Serious Problems

Ship sinking
Daily Mirror Archive/Mirrorpix/Mirrorpix via Getty Images
Daily Mirror Archive/Mirrorpix/Mirrorpix via Getty Images

It was at this moment that the captain and crew of the USS San Diego knew that they were in serious trouble. Once Captain Christy realized that all of the engines were shot, it was evident that there was little hope of them getting out of this situation.

The crew was unable to close off the damaged area, and the ship was being completely flooded inch-by-inch. When the vessel began tilting to the side, everyone on board knew the end was near. Every sailor knows this is a bad sign.

They Needed To Call For Help

Painting of ship sinking
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Willy Stöwer/ullstein bild via Getty Images

With every passing minute, the situation only continued to get worse. By this time, the water was not only flooding the lower decks but everywhere. The ship had taken on so much water that it began leaning at a nine-degree angle, and it was now as good as gone.

The water had swallowed the gun ports, entered the gun deck, and started pouring into coal chutes and air ducts. Captain Christy knew they couldn’t do anything on their own and had to call for help.

Even The Radio Was Down

Marines on the radio
Jacques Boyer/Roger Viollet via Getty Images
Jacques Boyer/Roger Viollet via Getty Images

When Captain Christie made his way to the radio room to send out a call for help, there was bad news waiting for him there too. The radio operator explained that the machinery powering their radio had been broken during the explosion.

Now, they were unable to contact anyone that might be able to help. However, a single hit shouldn’t have been able to take down all of the USS San Diego’s essential systems.

A Split-Second Decision

Men in a small boat
Universal History Archive/Universal Images Group via Getty Images
Universal History Archive/Universal Images Group via Getty Images

Through his experience, Captain Christy knew that his ship and crew wouldn’t have a chance surviving the explosion unless they got help. Considering that the radio was broken, he had few choices left and turned to his gunnery officer.

He instructed him to lead a small team and head to shore on a small boat and come back with help. As the crew watched the small boat make its way toward shore, they knew it was their only hope for survival.

It Began Sinking

Sinking ship
Three Lions/Getty Images
Three Lions/Getty Images

The explosion that hit the USS San Diego was so intense that the armored cruiser began to sink in just ten minutes. Knowing what must be done, he ordered his crew to lower the ship’s rafts and lifeboats.

Because the ship was one of the most prized vessels of the United States Navy, Captain Christy waited until he was one hundred percent positive that it was going to sink before he ordered to abandon ship.

All Hope Was Lost

Men jumping ship
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Universal History Archive/Universal Images Group via Getty Images

Even though Captain Christy wanted to wait until he knew it was all over, it was undeniable that the ship was lost. He was then forced to give the order that no captain ever wants to hear and instructed his crew to abandon ship.

Christy made sure to stay on board until he was positive that all of his crew had been evacuated safely as the boat slowly sank into the water. Soon enough, everyone was on lifeboats and rafts that were supposed to take them to safety.

More Than The Boat Was Lost

Men in lifeboats
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Daily Mirror Archive/Mirrorpix/Mirrorpix via Getty Images

After the explosion, it only took a mere 28 minutes for the USS San Diego to sink beneath the frigid waters of the Atlantic Ocean, not far from their destination. In less than a half-hour, the United States Navy had lost one of their most prized vessels. However, that’s not all that was lost.

When the ship left port, there were approximately 830 souls on board. Tragically, six of them would never make it back home. One fell overboard, another drowned, two died during the explosions, and two more died during the evacuation attempt.

Captain Christy Made It Out Alive

Lifeboats on the ocean
Universal History Archive/Getty Images
Universal History Archive/Getty Images

Captain Christy did the honorable thing and made sure that all of his men were off of the sinking ship before he dared to step off himself. However, he wasn’t one of the six crewmen that unfortunately lost their lives on that fateful day, although he almost was.

A crewman named Ferdinando Pocarboa was responsible for saving the captain’s life. He managed to pull Captain Christie from the ocean and place him inside a life raft.

Thankfully, There Was A Rescue Mission

Men in lifeboats
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ullstein bild/ullstein bild via Getty Images

As the Atlantic Ocean engulfed the USS San Diego, a rescue mission was underway. The gunnery officer and his team were successful and were able to reach the shore at Point O’ Woods, New York, in search of help.

There, they contacted Navy command, and rescue ships were sent to the scene at once. Even after Captain Christy and his crew reached shore, the ordeal was still far from over. There was still no evidence as to how the ship sank.

Questions Needed To Be Answered

Plane flying over water
Hulton Archive/Getty Images
Hulton Archive/Getty Images

Although the majority of the crew of the USS San Diego were safe and the ship was at the bottom of the sea, they weren’t going to let whoever did this get away with it.

The United States Navy just lost one of their most magnificent ships, so the United States Naval Air Service began looking for the potential submarine responsible for sinking it. The aircraft of the First Yale Unit was then tasked with scanning the seas around where the USS San Diego sank.

There Was A Mistake

WWI planes in the sky
George Rinhart/Corbis via Getty Images
George Rinhart/Corbis via Getty Images

Not long after taking to the skies, the Navy pilots were confident that they found the answer as to what sank the USS San Diego. When they thought they saw something peculiar in the area, they immediately started dropping bombs right away.

However, as it turns out, what they were dropping bombs on was actually the wreckage of the ship! This would be the start of the century-long investigation to discover what had sunk the massive cruiser.

The Investigation Begins

Men on a boat
Touring Club Italiano/Marka/Universal Images Group via Getty Images
Touring Club Italiano/Marka/Universal Images Group via Getty Images

Unfortunately, the Navy pilots were unable to answer what had caused the explosion. So, a month after the vessel sank, the Naval Court of Inquiry decided to conduct an in-depth investigation into the attack.

World War I was still largely underway, and an investigation seemed essential in case the enemy was making their way to American shores. While many people had their own theories about what had happened, little did anyone know that there wouldn’t be an answer for a hundred years.

Captain Christy Had His Own Theories

Men loading a torpedo
Hulton Archive/Getty Images
Hulton Archive/Getty Images

Being the captain of the lost ship, Captain Christy had his own theories about what was the cause of the explosion. There was no evidence that a submarine was in the area during the attack, and search parties never discovered some of the apparent signs that a torpedo had been fired.

Although Captain Christy had years of experience as a sailor, his theory that a torpedo had hit them was ruled out. At this point, the two explanations that were being investigated were mines or treason.

Naval Mines Were Now Under The Microscope

Illustration of naval mines
DeAgostini/Getty Images
DeAgostini/Getty Images

Since Christy’s theory that the ship was hit by a torpedo was deemed invalid, the investigative committee turned their attention to naval mines. Such technology had been around since the early 19th century and had been vastly improved by World War I.

Back then, these mines were explosive charges surrounded by vials of acid that there triggered when a ship’s hull bumped up against it. Free-floating mines are known as “drifting mines” whereas those anchored to the bottom of the ocean and more likely to hit a ship are called “moored mines.”

It Still Didn’t Make Sense

Painting of ship sinking
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Pinterest/Stefano style

Because no enemies were spotted in the area, mines seemed to be the most logical answer to what happened to the USS San Diego. Six of them had been found in the surrounding area, but things still didn’t add up.

If the ship had hit a mine, it most likely wouldn’t have detonated on the side of the vessel, but at the front. It was then decided that sea mines also were probably not the cause of the explosion.

It Must Have Been A Traitor

Picture of Kurt Jahnke
Library of Congress
Library of Congress

With torpedoes and sea mines being insufficient answers, there was still one theory that few people even wanted to consider, sabotage. Decades after the USS San Diego sank and remained at the bottom of the ocean, conspiracy theorists began talking about Kurt Jahnke.

Back in 1999, rumors began circulating the now known German spy and saboteur that had planted explosives aboard the ship. The theory wasn’t unrealistic either, as Jahnke was based in San Francisco, just like the USS San Diego was before leaving for the East Coast.

It Was Concluded That It Was Mines

Port side of the USS San Diego
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Despite investigating the matter, without much evidence, the Naval Court of Inquiry ended the investigation in August 1918. Because six sea mines had been discovered in proximity to where the USS San Diego sank, it was determined that they were responsible for the explosion.

Over a century later, some people still weren’t convinced by this conclusion. 2018 marked 100 years after the USS San Diego sank, and the United States Navy News Reserves announced that the cause of the explosion was still unknown.

People Wouldn’t Give Up So Easily

AGU meeting
U.S. Navy
U.S. Navy

The sinking of the USS San Diego was an undeniable tragedy, which is one of the reasons that experts refused to move on without an answer. While the first investigation didn’t result in any clear explanation, modern forensic and underwater technology might be able to help.

After conducting their own research, in 2018, the American Geophysical Union held its annual meeting. It was there that they revealed the answer that everybody has been waiting for.

The Final Verdict

Picture of Catsambis in lab
ASTRID RIECKEN For The Washington Post via Getty Images
ASTRID RIECKEN For The Washington Post via Getty Images

Alexis Catsambis, an underwater archaeologist for the Navy, decided to take a more in-depth look into the sinking of the USS San Diego. It was at the 2018 AGU meeting that he presented his findings.

Catsambis announced that he and other experts “believe that U-156 sunk San Diego” because the flooding patterns in the ship “weren’t consistent with an explosion set inside the vessel,” and it “didn’t look like a torpedo strike.”

The U-156

Picture of U-156
Pinterest/Ancestry Official
Pinterest/Ancestry Official

Much like any major army across the world, during World War I, the Germans had countless secrets that they wanted to keep hidden from the rest of the world. However, some of these secrets weren’t released to the public until recent years. One of them was the existence of an underwater submarine referred to as the U-156.

This submarine not-so-coincidentally was also operating along the south shore of Long Island. Catsambis and his colleagues then concluded that mines laid by the submarine are what laid waste to the USS San Diego.

Where Is The Ship Now?

Diver searching shipwreck
Allesandro Rota/Getty Images
Allesandro Rota/Getty Images

The wreckage of the USS San Diego can still be found off the coast of Fire Island, 110 feet underwater. It rests at a safe depth for people to dive and see for themselves, and has become one of the most popular destinations in the United States for shipwreck enthusiasts.

The ship is also upside-down, which makes it a dangerous place for inexperienced divers to investigate. Unfortunately, more divers have lost their lives than those who perished on board. It has also earned the nickname of the “Lobster Hotel” for the abundance of lobsters that have made it their home.

This Is Only The Beginning

Diver swimming through shipwreck
Andrey Nekrasov/Barcroft Media via Getty Images / Barcroft Media via Getty Images
Andrey Nekrasov/Barcroft Media via Getty Images / Barcroft Media via Getty Images

The story of the sinking of the USS San Diego and its investigation is something most people will find in their history books, no matter how interesting it might be. Furthermore, it only took researchers a century to even pinpoint exactly what happened on that fateful day.

Nevertheless, the USS San Diego is just one of the many ships that have been lost at sea with little understanding of why. As time passes, it’s possible we may learn the answers of many others.