Just minutes after crew members aboard the Australian submarine signaled about diminishing visibility, the vessel disappeared into the fog.

Early in World War I, Australia’s first submarine, HMAS AE1, and its sister ship, AE2, were employed in an operation to capture German New Guinea. It was a Sunday in September of 1914 when AE1 helped secure the surrender of Rabaul in East New Britain, according to the Royal Australian Navy.

But the next day, the boat and its crew vanished — leading to a 103-year-old maritime mystery.

“No trace of AE1 was found, not even the telltale shimmer of escaping oil floating on the surface of the water,” according to a historical account from the navy.

It says, “The loss of AE1 with her entire complement of three officers and 32 sailors was the [Royal Australian Navy’s] first major tragedy and it marred an otherwise successful operation to seize the German colonies in New Guinea and the South Pacific.”

The Australian government could not explain how AE1 disappeared, but it began the first of several new searches for the wreckage in 1976.

Australia’s Defense Department announced Thursday that the 800-ton submarine had been found, nearly 1,000 feet below the waves. The search vessel Fugro Equator located it earlier this week in waters east of Rabaul, not far from Duke of York Islands in Papua New Guinea.

“Australia’s first  HMAS  has been found after 103 years ending Australia’s oldest naval mystery,” Marise Payne, Australia's minister of defense, said on Twitter. “Congratulations to all involved & proud that Turnbull Government could support the search. Lest We Forget.”

Officials said that following the discovery, those on the search vessel held a commemorative service to honor the submarine’s crew.

The Australian government plans to work with counterparts in Papua New Guinea to try to preserve the site of the shipwreck, the navy says.

Brendan Nelson, director of the Australian War Memorial, called the find “one of the most significant and practically meaningful events of Australia’s commemoration of the First World War.”

The first significant Australian action of the war was the Australian Naval and Military Expeditionary Force’s (ANMEF) landing on Rabaul on 11 September 1914. The ANMEF took possession of German New Guinea at Toma on 17 September 1914 and of the neighbouring islands of the Bismarck Archipelago in October 1914. On 9 November 1914 the Royal Australian Navy made a major contribution when HMAS Sydney destroyed the German raider SMS Emden.

It added:

For Australia, the First World War remains the costliest conflict in terms of deaths and casualties. From a population of fewer than five million, 416,809 men enlisted, of whom more than 60,000 were killed and 156,000 wounded, gassed, or taken prisoner.

“The Great War, as it is often described, had a deep and lasting impact on our country,” Nelson said in a statement after the submarine was found. “The tragedy and loss felt by all Australians was palpable, and instances like the mysterious loss of one of our very first submarines greatly affected us. The successful location of AE1 is extremely important, and the nation should be grateful to all those involved.

“Now we can properly mourn the deaths of those men who served in AE1, and commemorate their sacrifice in a meaningful and fitting way,” he added.

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