Marathon swimmer Martin Strel to take on the Grand Canyon

Martin Strel has already smashed four Guinness World Records after swimming the entire length of the Amazon, Yangtze, Mississippi and Danube rivers.

Now the 56-year-old is setting his sights on conquering the Grand Canyon's Colorado River.

"The Colorado River on the Grand Canyon is America's most iconic river," he told BBC World Service's Outlook programme.

"It is very, very dangerous because it is used for rafting not for swimming. They have terrible rapids which I'm hoping to avoid because I don't want to die in Colorado."

Strel, dubbed the "fish man", is used to such death defying stunts after battling piranhas, bull sharks and pirates during his last challenge when he swam a 3,272-mile marathon in 66 days across the Amazon in 2007.

Extreme endurance

While the Colorado River is less than half the Amazon's length at a 1,450 mile stretch, the Slovenian swimmer believes it could be his most important swim yet primarily because he is keen to highlight the river's water shortages.

"My previous swims have all been about extreme endurance and stamina," he explained.

"This time I wanted to tackle a demanding clean, fresh water stream over a shorter distance. The goal is to show people how important it is to save such fresh, clean rivers which should stay alive and be part of our environment."

Wearing just a wetsuit and flippers during each swim, Strel has already put himself through a punishing training regime in the past. His latest hurdle will see him do the same.

"You have to train a lot, twice a day swimming, cross country, gymnastics, hiking" he explains. "It is difficult to say I will swim 5,000 kilometres but you must be very, very strong in your mind."

Strel set three previous long-swim records, starting with the 3,004km Danube in 2000, then the 3,797km Mississippi in 2002 and the 4,003km Chinese Yangtze in 2004.

While he managed to recover after all these swims effectively, psychologically he claims he is still recovering from his marathon across the Amazon.

"I still haven't really re-adjusted," he revealed. "Physically I'm OK but after the Amazon it was much harder to adjust than the others especially for my mind."

Treacherous waters

It is not surprising when you consider the challenges he endured.

Aside from swimming in treacherous waters filled with dangerous creatures, Strel battled with the sun, disease and less than five hours sleep a night as he swam an average of 52 miles a day.

"I didn't realise how much the sun could be such a big killer," he admits.

"My face was terrible even with the best medical team in the world, the doctors didn't know what to do with me.

"But you have to survive or die. Eventually my son found masks for me and a hat."

Strel said the key to his survival strangely involved a bottle of wine and the help of a few sea creatures.

"The best protection I had was from pink dolphins who swam alongside me every day, I think that was the best protection against bull sharks," he said.

"I also drank special wine which helped me a lot."

Strel will start his next marathon swim on the Colorado River in May 2011.