MIDWAY ATOLL, Northwestern Hawaiian Islands >> Deep-sea explorers scouring the world’s oceans for sunken World War II ships are honing in on debris fields deep in the Pacific, in an area where one of the most decisive battles of the time took place.

Hundreds of miles off Midway Atoll, nearly halfway between the United States and Japan, a research vessel is launching underwater robots miles into the abyss to look for warships from the famed Battle of Midway.

Weeks of grid searches around the Northwestern Hawaiian Islands have already led the crew of the Petrel to one sunken battleship, the Japanese ship the Kaga. This week, the crew is deploying equipment to investigate what could be another.

Historians consider the Battle of Midway an essential victory for the U.S. and a key turning point in WWII.

“We read about the battles, we know what happened. But when you see these wrecks on the bottom of the ocean and everything, you kind of get a feel for what the real price is for war,” said Frank Thompson, a historian with the Naval History and Heritage Command in Washington, D.C., who is onboard the Petrel. “You see the damage these things took, and it’s humbling to watch some of the video of these vessels because they’re war graves.”

Until now, only one of the seven ships that went down in the June 1942 air and sea battle— five Japanese vessels and two American — had been located.

The expedition is an effort started by the late Paul Allen, the billionaire co-founder of Microsoft. For years, the crew of the 250-foot (76-meter) Petrel has worked with the U.S. Navy and other officials around the world to find and document sunken battleships. It is illegal to otherwise disturb the underwater U.S. military gravesites, and their exact coordinates are kept secret.

The Petrel has found 31 vessels so far. This is the first time it has looked for warships from the Battle of Midway, which took place six months after Japan’s attack on Pearl Harbor and left more than 2,000 Japanese and 300 Americans dead.

The attack from the Japanese Imperial Navy was meant to be a surprise, a strike that would give Japan a strategic advantage in the Pacific. It was thwarted when U.S. analysts decoded Japanese messages and baited their enemy into revealing its plan.

As Japanese warplanes started bombing the military installation at Midway Atoll, a tiny group of islands about 1,300 miles (2,092 kilometers) northwest of Honolulu, U.S. forces were already on their way to intercept Japan’s fleet. U.S. planes sank four of Japan’s aircraft carriers and a cruiser, and downed dozens of its fighter planes.

One of the American ships lost was the USS Yorktown, an aircraft carrier that was heavily damaged and being towed by the U.S. on the battle’s final day when it was hit by torpedoes. The other, the USS Hammann, went down trying to defend the Yorktown.

Retired Navy Capt. Jack Crawford, who recently turned 100, was among the Yorktown’s 2,270 survivors.

Japanese dive bombers left the Yorktown badly damaged, with black smoke gushing from its stacks, but the vessel was still upright.

Then the torpedoes hit, Crawford told The Associated Press by telephone from his home in Maryland.

“Bam! Bam! We get two torpedoes, and I know we’re in trouble. As soon as the deck edge began to go under, I knew . she wasn’t going to last,” said Crawford, whose later military career was with the naval nuclear propulsion program. He also served as deputy assistant secretary for nuclear energy in the Department of Energy.

The Yorktown sank slowly, and a destroyer was able to pick up Crawford and many others.

In May 1998, almost 56 years later, an expedition led by the National Geographic Society in conjunction with the U.S. Navy found the Yorktown 3 miles (4.8 kilometers) below the surface.

Crawford doesn’t see much value in these missions to find lost ships, unless they can get some useful information on how the Japanese ships went down. But he wouldn’t mind if someone was able to retrieve his strongbox and the brand-new sword he left in it when he and others abandoned ship 77 years ago.

He was too far away to see the Kaga go down.

A piece of the Japanese aircraft carrier was discovered in 1999, but its main wreckage was still missing — until last week.

After receiving some promising sonar readings, the Petrel used underwater robots to investigate and get video. It compared the footage with historical records and confirmed this week it had found the Kaga.

The other three Japanese aircraft carriers — the Akagi, Soryu and Hiryu — and the Japanese cruiser Mikuma are still unaccounted for.

The Petrel crew hopes to find and survey all the wreckage from the entire battle, an effort that could add new details about Midway to history books.

Earlier this year, they discovered the USS Hornet, an aircraft carrier that helped win the Battle of Midway but sank in the Battle of Santa Cruz near the Solomon Islands less than five months later. More than 100 crew members died.

The Petrel also discovered the USS Indianapolis, the U.S. Navy’s single deadliest loss at sea.

Rob Kraft, director of subsea operations on the Petrel, says Allen gave him and his crew a mission to preserve history, educate people about the past and honor those who fought on these great battleships. Allen died last year.

“We’re still carrying on Paul’s legacy to honor our service members,” Kraft said. “This originated from his desire to honor his father’s service to his country, and to remember our service members and to make sure that future generations remember that as well, and they actually understand what that means and to respect that.”