It's been seven decades since Britain detonated the largest ever nuclear explosion on Australian soil, dwarfing the atomic bombs dropped on Japan in WW2.
Yet secrecy continues to shroud the exact force and fallout of the massive blast, triggered during the depths of the Cold War, on June 19, 1956.
It was in fact the final in a series of tests conducted on the remote Montebello Islands, 1400km north of Perth.
Recently declassified documents shed new light on the mystery of Operation Mosaic and G2 bomb but two reports remain under lock and key, controlled by the British Ministry of Defence.
"The British do keep their secrets," according to James Cook University researcher Elizabeth Tynan.
"The Australian government did not know what the British were doing and still to this day does not really know what was done on our territory."
Some estimates put the explosive strength of G2 as high as 98 kilotons or about six times that of the bomb that devastated Hiroshima in 1945.
But the official yield figure is 60 kT, just under the 62.5kT maximum Britain assured Australian authorities it would not exceed.
Three days after the blast, a top-secret message exchanged between British scientists working on the Mosaic project disclosed that G2's explosive yield may have been as high as 120kT.
Notably, it's estimated the cloud from the explosion rose to at least 14,000m rather than the predicted 11,000m. It eventually drifted inland and into the Arafura Sea, north of Darwin, almost 1500km away.
Documents also show the British never intended the G2 yield to be 62.5kT and were planning for about 80 or more but the actual figure can't be confirmed until the data is released, Professor Tynan tells AAP.
Such was the expected size of the blast that the same document suggested flying be suspended within a distance of 600 km for 10 hours.
"I don't know why the British won't come clean," she says.
G2 erupted a decade after Britain departed the US-led Manhattan Project in the wake of WW2.
Forced to forge its own path in a bid to maintain its status as a major power, it was intent on showing the Americans and Soviets it had mastered nuclear technology and could defend itself.
The program started with crude atomic bombs using nuclear fission but the aim was to develop a much more powerful thermonuclear hydrogen bomb using fusion technology.
Mosaic was a stepping stone and also involved the earlier G1 detonation, similarly using boosted fission weapons.
It was triggered in May 1956 and had a yield of 15kT. G2 used the same technology a month later but was wrapped in uranium to enhance its force.
Both bombs were dead-end designs used to rule out the technology and clear the way for a British doomsday H-bomb measured in megatons, or one thousand times a kiloton, to be tested in the Pacific a year later.
Defence force personnel who witnessed the events described a blinding flash of light, intense heat and a powerful blast-wave before the mushroom cloud rose into the sky.
Milton Ward who was an electrical mechanic first class in the Royal Australian Navy was ordered on deck on HMAS Tobruk in his summer uniform of shorts and a short sleeved shirt."They told us we had to turn our back, shield our eyes and the next thing this heat went straight through your body," he tells AAP."When it went off you were actually looking at an x-ray of your hands."He was also part of the clean up crew, picking up all the equipment to take it back to Leeuwin naval barracks in Fremantle."When I was on the Tobruk we didn't know what was going on," he says of the secrecy surrounding the testing.Afterward, Mr Ward says, while on board HMAS Karangi, they had British scientists on board.
"We were catching trevally and eating them, they had a Geiger counter and they ran it over the fish we were eating.
"You should have heard it go off."
Now 95, he carries scars over his face and neck, the result of numerous cancer removals."No-one ever told us how bad it would be or what the effects would be," he says.
"We just weren't told anything."
Jettisoning radioactive fallout into the atmosphere, the explosion left the pristine archipelago and surrounding marine environment with a persistent toxic aftermath.
Metal signs near the blast zones on Trimouille and Alpha islands now warn adventure-seeking visitors to spend no more than one hour in the vicinity.
In recent years, scientists have found plutonium remains in local marine sediment at levels up to 4500 times higher than other areas of the WA coast.
According to Ash Nesbit who runs commercial charters around the 174 islands, his passengers generally come for the world-class fishing and diving but are also keen to see the nuclear test sites.
"They can't believe the British actually did it," he says.
Rotting concrete bunkers, roads, a derelict command post, cabling and rusty machinery litter the low-lying islands.
"There's a huge crater in the water where they let one go off," Mr Nesbit says.
"You sort of drive along in 10 metres of water and then it drops down to 25 metres. It's really weird.
"I haven't seen any two-headed coral trout, though."
The 18 km-long island chain, about 120 km west of Dampier, is a maze of narrow channels between rocky outcrops covered in spinifex and scrub.
Its isolation, harsh climate and, of course, reputation mean just a few thousand hardy types annually dare explore its coves and coral reefs.
"It's just raw beauty and the British decided to set off a bomb; it's just ridiculous," Mr Nesbit says.
Now a conservation and marine park, the Montebellos were also the site of 12 nuclear weapons Britain tested in 1952, which then-prime minister Sir Robert Menzies agreed to without consulting his cabinet.
Operation Hurricane in 1952 detonated a 25kT fission device in the hull of frigate HMS Plym, which disintegrated, leaving a saucer-shaped crater on the seabed in Main Bay a few kilometres from the G2 site.
The tests then moved to Emu Field in the South Australian desert in 1953 for Operation Totem, with two detonations less than 10kT.
After G2, the testing returned to SA at Maralinga, with operations Buffalo in 1956 and Antler in 1957 producing seven detonations, the most powerful 26.6kT.
A Royal Commission into the tests in 1984-85 was scathing of Britain, Australia's compliance and the safety of the program.
It found fallout had spread across the nation, increasing cancer risks among the general population, and large tracts of land were contaminated.
Vulnerable Indigenous people near the test sites were displaced and many died due to exposure to radioactive material, if not the blasts.
Thousands of Australians, mostly defence personnel working on the tests, were also exposed.
Many wore no protective clothing and suffered higher cancer mortality rates and more cancers than the general population.
The inquiry also found the Montebello Islands were not an appropriate place for atomic tests due to the prevailing weather patterns.
The Mosaic tests were conducted in a hurry under marginal meteorological conditions, it said.
The commission accepted the G2 yield was 60kT.
Prof Tynan says Britain didn't see Australia as an equal partner and lied to it.
"Australia was a useful idiot," she says.
"They fed us lines ... they gave soothing words to make the Australian government feel it was all okay and all safe, when it really wasn't."
Prof Tynan's book Nuclear Archipelago: Secrets, power and the biggest atomic blast in Australia will be published in August.
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