Here’s what you’ll learn when you read this story:

• Shipwreck divers discovered an unknown German U-boat sunk off the coast of New Jersey.
• After dangerous investigations that led to the death of three divers, the team positively identified the submarine as the German-made U-869, originally believed sunk off the coast of Gibraltar.
• Just how the German U-boat and its crew of at least 48 came to rest in the depths of the Atlantic Ocean off New Jersey remains debated.

Sometimes an amazing discovery happens when you least expect it. That was certainly the case when, in 1991, a group of shipwreck-diving enthusiasts descended 230 feet into the icy, pitch-black waters of the Atlantic Ocean off the coast of New Jersey. The dive team was simply looking to explore an uncharted, anonymous wreck. Instead, they found a lost piece of history.

The team was led by an experienced shipwreck explorer named Bill Nagle, who operated a dive charter boat called the Seeker out of the small seashore town of Brielle, New Jersey. Nagle had learned about the shipwreck from talking to a local fishing boat captain in a Brielle dive bar called The Harbor Inn (affectionately known by locals at the time as “The Horrible Inn”). The fisherman knew about the wreck because his sonar had pinged it one day, but he’d kept the location a secret because it was a goldmine—every time he took clients to that specific spot, they caught an abundance of fish. However, commercial nets would occasionally snag on the massive object on the seafloor. So in exchange for the coordinates to another good fishing spot, the fisherman gave Nagle the coordinates of his secret ocean-floor anomaly.

According to a timeline provided by Rick Burton, Syracuse University professor and certified diver, when divers made their first descent from the Seeker to explore the mysterious seafloor site in 1991, they assumed they’d find an ordinary shipwreck. But according to an account from three of the divers, they saw the shadowy remains of what appeared to be a German World War II-era submarine.

Historical records offered no clues about the identity of the sunken vessel—or its crew. Getting answers required deeper investigation and many further dives. For years, divers continued to investigate the hazardous site without ever uncovering the identity or origin of the wreck, which they dubbed the “U-Who” because they didn’t know its real name. Sadly, the effort to discover the submarine’s identity came with a great cost. Three divers lost their lives over the course of several years of efforts to penetrate the wreck’s secrets, according to diver Richie Kohler.

What made the submarine so hard to identify was also what made it so dangerous. Because no exterior markings were visible, the only way to figure out the sub’s identity was to get inside it. But in the 1990s, deep-wreck diving technology was still in its relative infancy, and penetrating a collapsed World War II submarine 230 feet deep in the dark, freezing North Atlantic was widely considered a suicide mission. It took an extraordinary, life-threatening feat of underwater engineering and sheer physical grit to finally do it.

During the early years of exploring the wreck, divers used standard compressed air. But at 230 feet, breathing air causes severe, intoxicating nitrogen narcosis. To overcome that problem, diver John Chatterton and his team had to pivot to an experimental solution called Trimix—a mixture of oxygen, nitrogen, and helium. Because it was dangerous and unproven, the divers couldn’t buy Trimix at a local dive shop. Instead, they had to buy commercial tanks of pure helium and oxygen and manually blend the gases in their garages. Mixing high-pressure oxygen by hand carries a massive risk of spontaneous explosion if even a drop of oil or grease is present. And getting the proportions of the other gases wrong could easily kill a diver.

Breathing this dangerous mixture, Chatterton and Richie Kohler finally succeeded at getting inside the submarine in 1997. But it wasn’t easy. Fissures in the submarine’s hull had allowed its interior to fill with silt, and the slightest movement would cause it to rise up in the water, making sight virtually impossible inside the sub. So Chatterton and Kohler memorized German submarine blueprints on land so they could navigate the flooded compartments entirely by touch. To breach the most heavily obstructed compartments, they physically removed their air tanks and squeezed their bodies through narrow gaps in the collapsed bulkheads. This perilous choreography allowed Chatterton to reach the submarine’s electric motor room.

When the divers ripped three-inch long ID tags from a spare parts box in the motor room, they had to be careful not to disturb the remains of the roughly 50 German submariners still on board. After reaching the surface with the ID tags in hand, the dive team positively identified the Type IXC U-boat as the U-869. But that successful identification raised a fresh set of questions.

How the U-869 ended up at the bottom of the Jersey shore remains a subject of intense analysis. According to historical war reports, the U-869 was originally dispatched to the East Coast of the United States, but was later commanded via radio to change course and reinforce the German submarine fleet near Gibraltar. Because the German high command never heard from the U-869 again, an official Allied report from late February 1945, which claimed the French ship L'Indiscret and the American destroyer USS Fowler had combined to sink a submarine near Gibraltar, was never questioned. But the 1997 dive proved definitively that the U-869 never made it to its redirected destination.

So, how did the U-boat sink? Chatterton, Kohler, and John Yurga wrote that they believe the U-869 crew never received—or misunderstood—the radio message to head to Gibraltar and that an explosion on the port side of the control room sank the sub. Their working theory is that the U-869 fired a torpedo that failed to pick up its intended target, and instead it circled back to strike—and sink—the German U-boat it came from. But it’s just a theory.

Upon being offered the proof of the location of the U-869, the U.S. Navy crafted its own theory, giving the credit for sinking the submarine to the USS Howard D. Crow and USS Koiner, two destroyer escorts that were heading from New York to England on Feb. 11, 1945. In the Navy’s version of events, the two ships attacked a submarine they’d located on sonar with depth charges, resulting in bubbles and an oil slick, but no debris. But because they saw no movement or debris, the ships chalked up the sonar blip they saw to a previously sunken wreck, which could account for why there was no record of the “attack” in historical logs.

While each theory is at least partially plausible, “it is probable that we will never fully understand the specifics surrounding the sinking of the U-869,” the divers wrote. Still, the discussion couldn’t happen if not for the daring risks that divers took to explore the German U-boat off the shores of New Jersey—an 80-year-old mystery nobody even knew existed.