Investigators believe they have solved the 104-year-old mystery of what sank Australia’s first submarine and sent 35 sailors to a watery grave off Papua New Guinea.

Experts who analysed the wreck believe HMAS AE1 sank after a ventilation valve in the hull was left partially open when the sub dived.

It’s not clear if it was human error or a mechanical failure that caused the fault. However, with the valve open, water would have flooded the engine room.

As AE1 sank to its 100-metre crush depth, an implosion would have ripped through the vessel, killing all on board instantly, an Australian National Maritime Museum report says.

The submarine was the first wartime loss for the Royal Australian Navy and the first Allied submarine loss in the first world war.

It was last seen on patrol off the province of East New Britain on 14 September 1914 during an otherwise successful operation to seize the German colonies in New Guinea and the south Pacific.

A search by five navy ships in the days following failed to find the sub, nor any tell-tale shimmer of escaping oil floating on the surface of the water. And there was no distress call to help guide the search.

Enemy action was not suspected because the only German vessel nearby at the time was a small survey ship.

In 2017, the wreck was discovered under 300 metres of water during a search off the Duke of York Islands, near the former East New Britain capital Rabaul. It was the 14th attempt to find the vessel and the resting place of its crew.

AE1’s final contact with destroyer HMAS Parramatta at 2.30pm had placed it in the area.

Mioko Island villagers at the time also spoke of seeing a “monster” or “devil fish” that appeared and quickly disappeared back into the depths.

AE1 was one of two E-class submarines built in Britain for the new Australian navy.

AE2 achieved fame when it penetrated the Dardanelles waterway in Turkey at the same time Australian troops landed at Gallipoli.

After attacking Turkish shipping in the Sea of Marmara, AE2 came under fire from a Turkish gunboat and was scuttled so it would not fall into Turkish hands. All of its crew were captured and became prisoners of war. That wreck was discovered in 1998.