It will be the 74 years since the purpose-built diving bell - an observation chamber used by deep-sea divers to explore the depths of the ocean - embarked upon one of the greatest feats of underwater salvage at the time.

February 2, 1941, was the day the RMS Niagara, a 13,500-tonne ocean liner which sunk off the coast of New Zealand after striking a German mine eight months earlier, was located as part of a secret salvage operation.

The secrecy had to do with Niagara's cargo when it sank in June 1940; its strong room contained about $5 million worth of gold bars originally destined for the US to help pay for Australia's war efforts.

First considered futile by the Royal Australian Navy, the salvage mission kicked into gear when an Australian private salvage company contracted Melbourne engineer David Isaacs to design the bell and Thompson's Foundry in Castlemaine to build it.

Harcourt resident and former Thompson's Engineering Works employee George Milford said most of this information, however, was not known to the public until years later.

He said the diving bell was necessarily manufactured under great secrecy during a time when fear of espionage was rife.

"It was kept highly secret out at the factory," he said.

"Only those who had a need to know knew about it. They patrolled the factory at night because they were doing engineering for war."

Mr Milford said the diving bell was "purpose built in a great hurry" in October 1940.

"It was not generally known that a fair bit of Australia’s gold reserves had gone to the bottom (of the ocean)," he said.

"And so the hunt was on to recover that gold, but nothing leaked into the press for a long, long time, until the gold had been successfully recovered, because it could have damaged confidence when we were fighting the war."

Mr Milford said the diving bell itself was basically a cell with windows for the diver to look through and a telephone to communicate with the ship's captain.

"It looks like a giant mushroom; it’s got observation windows of very thick glass or Perspex to withstand enormous pressure and through that the diver could direct gold recovery operations into the strong room of the ship," he said.

"The diver made his descent and by telephone directed the grappling arms."

Much else of what is known locally about the diving bell comes from two sources: a book, Niagara's Gold, by Melbourne author Jeff Maynard and a collection of documents from diver John Johnstone's grand-daughter donated to the Castlemaine Historical Society.

John Conn, of Maldon Vintage Machinery Museum, said he had been entrusted with some of the engineer's drawings of the bell courtesy of author Jeff Maynard.

Mr Conn said he grew interested in the bell's history and got in touch with the author, who eventually passed on some of his research material.

"After a while he said, 'Right, this is what I’ve got, you're the guardian of it'," Mr Conn said.

Castlemaine Historical Society's Carol Dorman said the bell began testing from December 1940 on board the Claymore, an old coastal steam ship.

She said some of Mr Johnstone's notes from the testing period, including descriptions of a great storm and an incident on December 29 that saw the bell come into contact with a German mine, painted a picture of the inherent danger of the mission.

"They didn’t know if the windows could withstand pressure. John Johnstone got in and was bolted in, a horrifying prospect, and recorded details in his notebook," she said.

"He was down there writing in his book and making observations and all of a sudden heard  a sound he was not meant to hear."

Ms Dorman said when he investigated, Mr Johnstone discovered it was the anchor wire of a German mine.

"The bell was absolutely fastened to the mine, a bit scary and he knew if he went up the ship would go up and he would be no more, so there was a very lengthy operation to free the mine from the bell," she said.

"He goes into exhaustive detail but eventually ... they managed it successfully.

"John Johnstone's comments when he was finally hauled up were, 'I just did not feel I wanted to go below again, not for a couple of days anyhow'."

It was October 1941 before the Claymore crew blasted their way through the strong room door of the Niagara, at a depth of 420 feet, to begin recovering the gold.

Some records have operations ceasing two months later in December 1941 after a total of 555 gold bars had been recovered, worth an estimated $4,773,000.

Ms Dorman said that according to other records, it took them nearly a year to get most of the gold back up. Either way, Ms Dorman said the eventual recovery of the gold was likely to have been shrouded in secrecy.

"Because it was during the war and it was recovering 2,500,000 pounds of gold bullion, they wouldn’t have wanted many people to know about it," she said.

Records were also scarce when it came to how the bell ended back in Castlemaine, Ms Dorman said.

Mr Milford said it was hard to recall when the diving bell came back to Castlemaine, but thought it was in the early 1980s.

He said the bell's preservation in Castlemaine was important, especially as a piece of local engineering history.

"It was vital to the salvage operation and vital to our war effort," he said.

"People should be proud to know that Castlemaine produced this object, just as they should be proud to know Castlemaine produced two premiers of Victoria and also produced a lot of gold."