A French man has begun an extraordinary journey, swimming across the Pacific Ocean from Japan to the US all in the name of science and sustainability.

Ben Lecomte set off from Tokyo on Tuesday, headed for San Francisco over the next six months, which is an estimated 9,000 kilometres.

The 51-year-old, who lives in the US, has been preparing for more than six years for the swim, with his daily routine including hours of open water swimming.

He also said he practised "visualisation and dissociation" exercises to ensure he is mentally ready.

"The mental part is much more important than the physical," he told AFP news agency.

I remember my father and he was the one who taught me how to swim in the Atlantic. I remember times when we would go on the beach and walk and never see any plastic. Now, everywhere I go, on the beach I see plastic everywhere -Ben Lecomte

With his journey only just beginning, he knows the dangers he faces which include sharks, storms, swarms of jellyfish, and extremely low water temperatures.

"You have to make sure you always think about something positive,” he says.

"When you don't have anything to occupy your mind it goes into kind of a spiral, and that's when trouble starts.”

Accompanied by a nine-person support team, Mr Lecomte is hoping to raise awareness for climate change as his team of scientists conducts research during the six-month swim.

They will study plastic debris, the effect of extreme exercise on the heart and examine how the Fukushima nuclear disaster has affected the ocean.

More than six years of preparations have lead to this moment. Finally really to start my swim across the Pacific Ocean. #theswim #benlecomtetheswim watch the live stream of my departure on @Seeker Facebook. @Discovery pic.twitter.com/TPJlz4tbEY — Ben Lecomte (@BenLecomteSwim) 4 June 2018

For Mr Lecomte, raising awareness of these issues is more important than setting any record.

“I remember my father and he was the one who taught me how to swim in the Atlantic. I remember times when we would go on the beach and walk and never see any plastic. Now, everywhere I go, on the beach I see plastic everywhere,” he says.

Mr Lecomte is no stranger to a swim of this nature. In 1998, he made the first known solo trans-Atlantic swim covering 6,400km in 73 days.

READ MORE Police arrest 12-year-old boy trying to drive solo across Australia

His first word were “never again” when he reached dry land in France, but it wasn’t long before he started looking for a new challenge.

"It didn't take that long for me to change my mind," he told NPR.

"Three, four months afterwards I was already thinking about my next adventure and doing something kind of the same."

His daily routine will remain virtually the same for the entire trip. He will swim for eight hours, jump on board the support boat to eat and sleep, and then enter the water again.

Keeping his energy levels high will also be a challenge for Mr Lecomte, as he aims to consume 8,000 calories a day.

"I have a strict no sugar diet. Most of my calories will come from a high-fat diet and include a lot of freeze-dry full meals, rice, pasta and various soups," he posted during a Reddit interview earlier this month.

A GPS tracker will be fitted to his support boat to allow interested viewers to keep up-to-date with his progress on his website.