Tim Taylor was about to end the mission. His team had scoured the seabed off Japan with autonomous underwater vehicles, which are essentially high-tech drones, without a hit. His ship now needed repairs, and a $7 million drone had just reported an error on its latest dive.

All that remained was to download the data from that drone before heading hundreds of miles back to shore.

That’s when they spotted it: An anomalous reading on the ocean floor, more than 1,400 feet deep. The next day, another submersible with high-definition cameras went to investigate.

The images it beamed back left no doubt about what Taylor’s team had found: A hulking ship lay rusting in the pitch-black water. As the camera rounded the bow and panned to the bridge, an eerily preserved plaque came into view: USS Grayback.

“It was amazing. Everyone was excited,” Taylor said in an interview with The Washington Post. “Then you realize there are 80 men buried there, and it’s a sobering experience.”

Taylor’s discovery on June 5 solved an enduring 75-year-old mystery about the fate of the USS Grayback, one of World War II’s most effective submarines. The U.S. Navy confirmed on Sunday that Taylor’s team, part of a group dedicated to finding the 52 American submarines lost in action during World War II, found Grayback’s final resting place in the ocean off Okinawa, Japan.

The news brought closure to relatives of the sailors lost that day.

“There’s a book I read, and it said these ships are known only to God,” Gloria Hurney, whose uncle, Raymond Parks, died on the Grayback, told ABC News. “But now we know where the Grayback is.”

The Grayback’s final mission started on Jan. 28, 1944, according to the Navy’s official history, when it left Pearl Harbor on its 10th combat tour. Commissioned in 1941, the Tambor-class sub had spent the war patrolling the South Pacific and South China Sea, torpedoing numerous enemy vessels and rescuing downed American aviators. The Grayback sank more than a dozen Japanese ships in all, the New York Times reported.

On Feb. 24, 1944, the sub reported sinking two Japanese cargo ships days earlier and was ordered back to replenish its torpedo supply. But it never arrived in Midway.

After the war, the Navy used Japanese military records to try to piece together a history of its lost subs, and pinpointed the submarine’s final resting place as about 100 miles east-southeast of Okinawa, the Times reported.

But its remains were never found — until Taylor took on the case.

In 2010, Taylor, an undersea explorer and CEO of a New York-based firm that provides autonomous underwater vehicles, discovered the USS R-12, which sank in an accident off Key West, Fla., in 1943. He set up a privately funded group called the Lost 52 Project, dedicated to using new technology to find long-lost World War II subs. Along with his wife, fellow explorer Christine Dennison, his team found three more vessels before tackling the Grayback.

In this case, he relied on a key discovery by Yutaka Iwasaki, a systems engineer in Kobe, Japan, who works with Taylor’s team as an amateur researcher. Last year, while poring over original Japanese military documents, he found reports showing that on Feb. 27, 1944, a Japanese aircraft had dropped a 500-pound bomb on the Grayback. The coordinates given in that report suggested the Navy had made a crucial error when translating the coordinates where the sub was attacked.

“It was off by one digit,” Taylor said. “That changed the location by more than 100 miles.”

Armed with the correct information, his team journeyed in June into the open seas off Japan with a fleet of the latest submersible drones. The devices use sonar to create detailed images of the ocean floor, which are then sent back to Taylor’s crew to search for any signs of wreckage, the Times reported.

The search team battled technical problems, though, and a broken refrigeration system on his boat would soon require a return to port for a fix. Some crew members were already packing up to return when the drone experienced an error a few hours into a 24-hour dive, Taylor said.

Instead, the data it returned led Taylor’s team directly to the sunken sub.

“The most compelling moment was when the camera went from the bow up to the bridge and we all saw the plaque,” he said. “There was no doubt about it.”

After the Navy verified the find, Taylor and Dennison informed relatives of the men lost on the Grayback that they had found their final resting place.

Hurney and Kathy Taylor, whose uncle John Patrick King was on the sub, learned about the discovery live on camera on ABC’s “World News Tonight” on Sunday. Both women wept and thanked Taylor’s team.

“I committed from the very beginning when I was a little girl that I was going to find him or follow him or keep his memory alive, whatever I could do,” said Kathy Taylor, who isn’t related to Tim Taylor.

Navy officials hailed Taylor’s discovery for closing a long-open chapter for the submarine and its crew.

“Each discovery of a sunken craft is an opportunity to remember and honor the service of our sailors,” Robert S. Neyland, the head of the Naval History and Heritage Command’s Underwater Archaeology Branch, said in a statement. “Knowing their final resting place brings closure, in some part, to their families and shipmates as well as enables our team to better understand the circumstances in which the boat was lost.”

Taylor said he hopes his project casts more light on the thousands of sailors who died on submarines, many of whose heroic exploits were classified until decades after World War II.

“It’s a missing piece of their family stories and their legacy,” Taylor said. “It haunts these families for their whole lives. It’s very sad but rewarding to show these people finally where their loved ones sacrificed their lives.”