We all know Bear Grylls is up for a challenge. But what happens when he's the one setting the challenge for others?

As he tells news.com.au, it's not good for those who volunteer to take part.

Grylls, the former SAS serviceman and wilderness survival expert, has invented the world's most gruelling physical challenge – a non-stop, 11-day race covering 671km of rugged Fijian terrain that asks competitors to climb mountains, fight their way through jungles and paddle across oceans while averaging less than two hours sleep a night.

Grylls casually exits a chopper during filming in Fiji. Photo / Supplied
Grylls casually exits a chopper during filming in Fiji. Photo / Supplied

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Working with Survivor creator Mark Burnett, Grylls is bringing back the World's Toughest Race: Eco-Challenge, an expedition adventure race which last took place in 2002 before an 18-year hiatus.

The challenge is unlike anything else out there. But don't take our word for it. The trailer for the show, which will premiere on Amazon Prime Video in August, says it all.

In all, the challenge includes 330 competitors who make up teams of four or five from around the world, including a number of teams from Australia.

Producers of the 10-episode event say "the greatest challenges are not the competing teams, but the unforgiving 671km of terrain which stands between all those competing and the finish line".

"Viewers worldwide will see the limits of human physical and mental endurance tested like never before," they said in announcing the Australian premiere on Tuesday.

News.com.au spoke with Bear Grylls ahead of the announcement. He said the race is like nothing he has worked on before.

"It's the most extreme adventure race ever staged in human history," he said. "To run something like 671km non-stop. It's unbelievable."

He said competitors would sleep for just 15 hours total across 11 days.

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"We have to force people to sleep or else they'll die," he told news.com.au.

"At the end of this, you're going to see the best and the worst of these competitors."

The return of the Eco-Challenge is something Burnett has been planning for years. He approached Grylls about taking part, but the pair agreed it would only work if Grylls runs things himself.

"He said to me, 'I want you to make this bigger, badder, harder than before.' I told him, 'I'm really going to set the bar at a crazy level.' And this is the result."