In short:
The Backyard Ultra World Team Championship involves runners completing a 6.7-kilometre course loop on the hour, every hour, until one athlete remains.
Only two Australians out of a team of 15 remain in the event at Kembla Grange, having clocked more than 1,000 laps.
The Australian pair hope to reach 100 hours by Thursday morning.
It sounds like a metaphor for torture: a running race with no finish line.
About 900 runners from 67 nations, including 15 Australians, crossed the start line of the Backyard Ultra World Team Championship at 11pm on Saturday AEDT at Kembla Grange in the New South Wales Illawarra.
In a deceptively simple test of physical and mental grit, each athlete completes a 6.7-kilometre course loop on the hour, every hour, until one runner remains.
They squeeze food and minutes of sleep into the remaining time, in an event that can stretch beyond four days.
Jonathan Ash was the first Australian to pull out, succumbing to injury on Monday afternoon after he exceeded the hour limit on his 41st lap.
He told the event's live stream that knee bursitis and muscle tightness from the 16th lap slowed him down.
"Basically every muscle in my body started to get knotted at some point," Ash said.
"We had to get a physio in to push that out.
"[It was] probably one of the most painful experiences of my life."
The Shellharbour engineer is the second-ever Australian to complete 100 laps, equal to 670 kilometres, over four days in 2024, behind Phil Gore, who sits injured on the sidelines this year, tending to fellow athletes.
Gore broke the world record last year after running 119 laps, or 797.3km, after five days on a southern Queensland farm.
Melbourne's Brandon Lee Gretch tapped out after lap 73 about midnight on Wednesday and said he was proud of his effort, having achieved a personal best.
"I came in with 20 seconds to spare and had a shooting pain down my back. That was just killing me from about yard 67 onwards," he said.
"It was hell."
The 32-year-old was cheerful as he snacked on chicken nuggets this morning with heavily bandaged feet.
"Myself as an eight-year-old would be so proud to know I got to put on the green and gold,"Gretch said.
"It's weird because I am so sleep-deprived, I feel like my body automatically goes into recovery mode.
"The only bad thing is I always wake up [with] stiff legs, so I'll have to get moving again soon."
Backyard ultras were invented 15 years ago by Gary "Lazarus Lake" Cantrell and hosted at his Bell Buckle property in Tennessee.
"He wanted to challenge people to complete 100 miles in 24 hours," Australian race director Red Barnett said.
"It just grew in popularity as people pushed beyond the 24-hour mark.
"It is a bit of a crazy event."
Ash described the event as "the lesson that every person is incredibly capable of much more than they think they are".
"You don't know until you give it a shot," he said.
Australian athletes have so far completed more than 1,000 laps, but the attrition rate has been severe.
"There's a mixture of fatigue, not getting their nutrition right, body ailments that are going on," Ms Barnett said.
"So unfortunately, we just had one after the other [drop out] and are left with two [Australian competitors].
"If all goes well, these guys [Dan Camac and Ben Nicholls] are hoping to get to 100 hours, which is Thursday morning."
Ms Barnett said things would become "drastically harder" for the remaining Australians.
"When there is only two left, they really need to work together," she said.
"Having a lot of people around to support and cheer them on, even through the nights, all that really helps, but it's going to be a huge mental test."
