It's a story that's captivated treasure-hunters for decades — an enormous horde of riches that simply disappeared from the face of the earth.

And two decades after the plane carrying that vast wealth crashed into the sea near a sleepy Canadian fishing village, the mystery is no closer to being solved.

It was the night of September 2, 1998 when SwissAir Flight 111 went down, just two hours after taking off from New York, bound for Geneva.

The plane hit the water, breaking up on impact 8km from the tiny Nova Scotian communities of Peggy's Cove and Bayswater.

All 229 passengers and crew members died in the wreckage, and the plane was shattered into countless pieces.

But among the human tragedy of the disaster, rumours of another staggering loss soon emerged.

According to The Canadian Press, also on board that flight was unimaginable wealth, including "more than five kilograms of diamonds and jewels", a Picasso painting "worth millions" and "nearly 50 kilograms in cash".

None of it has ever been recovered.

Journalist and author Stephen Kimber, who wrote the book Flight 111: A Year in the Life of a Tragedy, told The Canadian Press talk of the treasure — worth a combined total of more than $AU528 million, according to a 2011 CBC News report — began swirling almost immediately after the accident.

"There was a lot of talk about it after the crash, that there had been all these valuables on board. That was a big deal," he told the outlet.

"Somewhere down at the bottom of the ocean, theoretically, are those diamonds."

And documentary producer John Wesley Chisholm, who also spoke with The Canadian Press, said while treasure hunting around the wreckage had been officially banned, it was possible people had managed to swipe some of the loot by obtaining treasure trove licences under the guise of searching other, nearby shipwrecks.

"A very common treasure-hunting technique is to say, 'Oh yeah we're looking for this wreck over here' … where they may in fact have been looking for the Swissair treasure," he told the news agency.

"Treasure, it just makes people crazy … Somehow, it just pulls on the psyche of men to do crazy things.

"The notion that there could be $300 million of diamonds just there, out of sight, just away from where everyone is, is just an absolutely irresistible pull for a certain kind of person."

Mr Kimber claims the plane was carrying a diamond that was part of a special exhibition from the American Museum of Natural History along with one kilo of other diamonds, 4.5 kilos of other jewellery, 49 kilos of cash and a Picasso original, believed to have been destroyed on impact, titled Le Peintre and worth over two million dollars.

After the crash, a special ship with an in-built vacuum was taken to the site in an attempt to salvage parts of the wreckage — but while 18,000kgs were recovered, there has never been any mention of any valuables found.

And plans by insurance company Lloyd's of London to search the area for the jewels were quickly scrapped following an outpouring of anger from the victims' families.

Earlier this month, hundreds of those friends and relatives of Flight 111 victims gathered for a moving ceremony at Bayswater alongside community members and first responders to mark the 20-year anniversary of the tragedy.

Among them was American Robert Kokoruda, who lost both his parents and four friends in the disaster.

"Seems like yesterday," he told CBC. "It's just hard to believe it's 20 years."

The exact cause of the crash is still unknown, although a Transportation Safety Board of Canada (TSB) investigation revealed 11 causes and contributing factors — including the fact that "aircraft certification standards for material flammability were inadequate", allowing "the use of materials that could be ignited and sustain or propagate fire".

Once a fire started on board, it spread rapidly, damaging aircraft systems and ultimately leading to "the loss of control" of the plane.

The victims included 132 Americans, 41 people from Switzerland and 30 from France as well as residents of the UK, Germany, Italy, Canada, Greece, Lebanon, Afghanistan, China, India, Iran, Russia, Saudi Arabia, Spain, St Kitts and Nevis, Sweden and Yugoslavia.

The former head of the World Health Organisation's AIDS program, Jonathan Mann, also died that night alongside his wife, AIDS researcher Mary Lou Clements-Mann.