Weather records from the 1920s may finally put to rest questions over whether Sir Edmund Hillary and Tenzing Norgay really were the first people to reach the top of Mt Everest.
It's one of mountaineering's most enduring mysteries - did the two British climbers who died during a 1924 expedition to the top of Mt Everest beat Sir Ed to the summit?
British climber and author Graham Hoyland has made nine trips to Everest searching for the remains of George Mallory and Andrew 'Sandy' Irvine.
"When your Ed Hillary got to the summit of Everest, he didn't dance around and plant a flag, the first thing he looked for was signs of Mallory and Irvine," Hoyland told Newshub.
Mallory and Irvine were last seen about 240m below the peak before disappearing into the clouds.
Mallory's body was discovered in 1999, while Irvine's was never found.
Items like broken climbing rope and goggles have been recovered, but the camera it's believed was lent to Mallory by expedition member Howard Somervell was never found.
That camera might have held evidence that the pair had summited, Hoyland said.
"I started the search for Mallory in 1999, we found his body but there was no camera."
Multiple expeditions since then have come no closer to an answer, but now, Hoyland believes a crucial clue is in the barometer records of his distant relative Somervell, who was in charge of monitoring the weather on the expedition.
"There was a ten millibar drop in pressure as Mallory and Irvine neared the summit, that is the equivalent of 180m extra on the summit," Hoyland said.
He says that represents an "invisible enemy" for climbers.
"Mallory and Irvine would have seen the blizzard coming in and they would have felt the pressure drop because they would have become breathless because they're breathing ambient air as well as the oxygen, and I'm pretty sure they turned back just before the second step."
So what does Sir Ed's family think of it all?
Peter Hillary said his father and Tenzing Norgay saw no evidence of Mallory and Irvine having reached the summit.
"The overwhelming evidence is that they didn't get up there," he said.
"The second step was a rock climbing feature that would have been technically too difficult for people at that time, in fact most people don't climb it now, they climb a ladder.
"They were late in the day, they were about to run out of oxygen, so the evidence isn't good.
"But it will always be one of those enduring mysteries, and I can tell you that Ed Hillary always had a lot of respect for George Mallory."
And he said the climbers of the early 1920s were an incredibly bold bunch.
"They had to make do, they had hobnail boots, long woollen stockings, they just had woollen coats, sports coats!"
Peter Hillary said a journalist phoned and put it to Sir Ed back in 1999 "and dad just replied, 'Oh well, I've had 40 good years out of it'."
A Kiwi response to a question that's captivated climbing buffs for a century.
