By Jamie Seidel

It was the US Navy's greatest tragedy: hundreds of survivors of the stricken USS Indianapolis were taken by sharks. Now, 72 years later, we're about to find out why.

Researchers led by US billionaire Paul Allen have announced the discovery of one of America's most significant wrecks.

The USS Indianapolis has passed into legend with its tale of delivering the "Little Boy" nuclear bomb to be dropped on Hiroshima, an early morning ambush - and a bungled search and rescue effort, reports News.com.au.

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Sunk by Japanese torpedoes in July 1945, the harrowing four days where survivors in the water struggled against sharks passed into immortality when the story was told in the Steven Spielberg blockbuster Jaws.

The USS Indianapolis had until that point been something of a lucky ship. It narrowly missed being at Pearl Harbor the day of the surprise attack that catapulted the United States into war. It fought successfully through many of the following Pacific campaigns.

But on the early morning of July 30, 1945, all that changed.

Shark attack

The heavy cruiser USS Indianapolis carried a crew of about 1200. Some 900 made it into the water after torpedoes from a Japanese submarine sent it beneath the waves in less than 12 minutes.

Starved. Sunburnt. Dehydrated.

The survivors had escaped with little more than their lifejackets.

A spare parts box from the USS Indianapolis on the floor of the North Pacific Ocean. Photo / AP
A spare parts box from the USS Indianapolis on the floor of the North Pacific Ocean. Photo / AP

For three and a half days, they clung desperately to debris from the sunken warship. There had not been enough time to deploy the life gear.

All the while the sharks circled. One by one, they took their prey.

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It would become the worst shark attack in history.

The USS Indianapolis' final desperate signals for help were received. But US intelligence officers - not certain of the ship's secret movements - believed the message to be a Japanese trap.

When a rescue mission was finally mounted three days later, the survivors were near death.

Only 317 men were pulled from the water.

Just 22 remain alive today.

Lost at sea

The bottom of an anchor, marked 'U.S. Navy' and 'Norfolk Navy Yard' belonging to the USS Indianapolis. Photo / AP
The bottom of an anchor, marked 'U.S. Navy' and 'Norfolk Navy Yard' belonging to the USS Indianapolis. Photo / AP

USS Indianapolis has been the subject of incessant controversy and debate since the end of the war.

Its commander, McKay, was court-martialled and blamed for not trying to dodge the torpedoes. He suicided in 1968. His conviction was overturned in 2001.

The exact location of the wreck has remained a mystery. All attempts to find her have failed.

Until now.

"For more than two decades I've been working with the survivors. To a man, they have longed for the day when their ship would be found, solving their final mystery," said Captain William Toti (Ret), spokesman for the survivors of the USS Indianapolis.

It took a methodical search over a 1200 square kilometre search area before the ship was found. And below the choppy surface was a 2500m tall mountain-range, full of cliffs and valleys.

The research ship Petrel and its robotic undersea rovers were guided by clues in the historical record, and the accounts of sailors themselves, to the most likely resting place.

Information uncovered in 2016 by Naval Heritage Command historian Dr Richard Hulver sent the search to the west of the previous estimate of the warship's final resting spot.

Exactly where that ended up being remains a tightly guarded secret.

The wreck of the USS Indianapolis, sitting 5500m beneath the surface, is surprisingly pristine.

The ship's identification number is clearly visible. Its camoflague paint remains crisp and clear. Little rust stains its decks.

Timeless memorial

Wreckage of the USS Indianapolis, including the ship's bell at the bottom of the North Pacific Ocean. Photo / AP
Wreckage of the USS Indianapolis, including the ship's bell at the bottom of the North Pacific Ocean. Photo / AP

Paul Allen has made an art of finding lost warships. He located the Japanese super battleship Musashi in 2015 and an Italian destroyer earlier this year.

He also recovered the ship's bell from HMS Hood - the Royal Navy's greatest tragedy of World War II.

Now the images of USS Indianapolis are revealing how the pride of the US Navy sank so quick.

Enormous holes have been found in the ship's side.

But the ship's guns remain proudly in place.

"To be able to honour the brave men of the USS Indianapolis and their families through the discovery of a ship that played such a significant role during World War II is truly humbling," Mr Allen said in a statement.

"As Americans, we all owe a debt of gratitude to the crew for their courage, persistence and sacrifice in the face of horrendous circumstances."

He hopes to compile a detailed report of the wreck's condition, and why it took just 12 minutes to sink.

"Even in the worst defeats and disasters there is valour and sacrifice that deserve to never be forgotten," said Sam Cox, Director of the Naval History and Heritage Command.

"They can serve as inspiration to current and future Sailors enduring situations of mortal peril.

"There are also lessons learned, and in the case of the Indianapolis, lessons relearned, that need to be preserved and passed on, so the same mistakes can be prevented, and lives saved."