Deep-sea explorers scouring the world's oceans for sunken World War II ships are investigating what they believe could be the third ship of seven lost to the Pacific during the Battle of Midway.

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 warship, the Japanese ship the Kaga.

Director of subsea operations on the Petrel, Rob Kraft, left, looks at images of the Japanese aircraft carrier Kaga, off Midway Atoll in the Northwestern Hawaiian Islands

The Japanese aircraft carrier Kaga is shown in the Pacific Ocean off Midway Atoll in the Northwestern Hawaiian Islands

Deep-sea explorers scouring the world's oceans for sunken World War II ships are honing in on a debris field deep in the Pacific

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.

Frank Thompson, a historian with the Naval History and Heritage Command in Washington, D.C., who is onboard the Petrel said: '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.

'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.'

A research vessel called the Petrel is launching underwater robots about halfway between the U.S. and Japan in search of warships from the Battle of Midway

Weeks of grid searches around the Northwestern Hawaiian Islands already have led the Petrel to one sunken battleship, the Kaga

This week, it's investigating what could be another. The other three Japanese aircraft carriers - the Akagi, Soryu and Hiryu - and the Japanese cruiser Mikuma are still unaccounted for

Director of subsea operations of the Petrel, Rob Kraft looks at images of the Japanese aircraft carrier Kaga, off Midway Atoll in the Northwestern Hawaiian Islands

Rob Kraft, left, and the Naval History and Heritage Command's Frank Thompson, left, look at footage of the Japanese aircraft carrier Kaga

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

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 ships.

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.

Retired Navy Captain Jack Crawford poses for a photo at his home in Rockville, Maryland. Captain Crawford, who recently turned 100 years old, served on the USS Yorktown during the World War II Battle of Midway and survived the Yorktown's sinking

Retired Navy Captain Jack Crawford signs an autograph at the Battle of Midway dinner at the Army-Navy Club in Arlington

As Japanese warplanes started bombing the military installation at Midway Atoll, a tiny group of islands about 1,300 miles (2,090 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.

The shoreline of Midway Atoll in the Northwestern Hawaiian Islands is shown from a landing airplane

A sign is shown on a damaged World War II building on Midway Atoll in the Northwestern Hawaiian Islands

A damaged World War II radar station is shown on Midway Atoll in the Northwestern Hawaiian Islands

A damaged World War II building is shown on Midway Atoll in the Northwestern Hawaiian Islands

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 said Captain Crawford: '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.'

Captain Crawford's 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.

An aerial photo of a Japanese carrier manoeuvring in a complete circle in an effort to escape in the Midway Islands, Hawaii

Crewmen aboard the USS Yorktown battle fire after the carrier was hit by Japanese bombs, during the Battle of Midway. After the battle, the Army reported repeated bomb hits on the enemy carriers Kaga and Akagi, while the Navy, in listing results, said four enemy carriers were definitely sunk

The Yorktown sank slowly, and a destroyer was able to pick up Captain 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 (5 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.

In this May 1942 file photo a Japanese heavy cruiser of the Mogami class lies low in the water after being bombed by U.S. naval aircraft during the Battle of Midway

USS Yorktown listing heavily to port after being struck by Japanese bombers and torpedo planes in the Battle of Midway

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.

June 4 1942 photo provided by the U.S. Navy shows a scene on the flight deck of USS Yorktown shortly after it was hit by two Japanese aerial torpedoes

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 honour those who fought on these great ships. Allen died last year.

Mr Kraft said: 'We're still carrying on Paul's legacy to honour our service members.

'This originated from his desire to honour 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.'

June 4, 1942 photo shows Japanese Type 97 shipboard attack aircraft from the carrier Hiryu amid heavy anti-aircraft fire, during the torpedo attack on USS Yorktown (CV-5) in the mid-afternoon

USS Astoria (CA-34) steams by USS Yorktown (CV-5), shortly after the carrier had been hit by three Japanese bombs in the battle of Midway