London, England (CNN) -- For polar explorer Ann Daniels, the worst part of this year's expedition to the Arctic won't be enduring bitterly cold temperatures or pulling a 100-kilogram (220-pound) sledge over steep jagged ridges.
It will be the moment she leaves the ice to swim through dark, icy waters with the threat of polar bear attacks preying on her mind, and worse.
"Nothing from under the sea is going to jump up and eat you. But as a human being there's that feeling of, 'what is under here? It's pitch black and anything can get me.' Mentally you start imagining all kinds of things in the water," she said from her home in Devon, southwest England.
Daniels left the United Kingdom this week with two other British explorers, photographer Martin Hartley and Charlie Paton, for a 500-kilometer (310-mile) trek towards the North Pole as part of the Catlin Arctic Survey's 2010 expedition.
After taking time to acclimatize to the subzero temperatures, they will be dropped onto the sea ice more than 1,000 kilometers off the Canadian coast.
From there they will walk and swim around 10 kilometers a day for the next 50 days in temperatures as low as minus 75 degrees Celsius, including windchill, while pulling a sledge of supplies 1.5 times their own bodyweight.
Along the way, they will drill holes in the ice to collect water samples so scientists can examine the levels of carbon dioxide in the water over the Arctic winter into spring.
"It's very tough mentally to survive and travel, and that's all that most adventurers do. Our team has to provide a professional survey service on top. That's a huge ask and inevitably it pushes them to their absolute limits," said the survey director and arctic explorer Pen Hadow.
Ann has been preparing for the expedition for the last six months, hauling a heavy tire through a field to build up the necessary muscles while continually reminding herself why she puts herself through the ordeal.
"I don't have any scientific background but what I can do is pull a heavy sledge through very difficult conditions," she said.
"To be able to help the scientific community understand what's going on in a part of the world that I truly love, then it's worth going through all the difficulties. If I can help the Arctic territory, or indeed any part of the world that we live in, that's such an honor for me."
The British mother of four is one of the world's most respected polar explorers. She has been to the Arctic at least seven times since 1997 and says each year the changing landscape has made the journey ever more treacherous.
"When I first went up in 1997, there were many more areas of really thick ice, bigger ridges, there were less areas of open water. We didn't have to do any swimming at all," she said.
"The first time I swam was in 2002 and steadily we've had to swim more and more as the years go on, and we're certainly expecting this year to do an awful lot of swimming. We've got a flotation device to go around the sledge as well because we're expecting more water than ever before."
Daniels is in no doubt that the Arctic ice is melting. She was part of the 2009 Catlin Arctic Survey which collected data on ice thickness that led scientists to conclude that the Arctic Ocean could largely be ice-free during summer in as little as 20 years.
The focus of this year's trip is on ocean acidification, which refers to the increasing acidity of the ocean as it absorbs more carbon dioxide from the atmosphere.
Early research shows higher ocean acidity is likely to affect the ability of some marine organisms to survive.
Last year's trip lasted 74 days. This year, the team will be airlifted out only when the spring ice-melt makes it too dangerous to stay.
Daniels says her children -- 15-year-old triplets and her six-year-old daughter -- have become accustomed to her leaving for long periods of time, and even urge her to go.
"I ask them now: 'Look this has come up, what do you think about it?' They have been so supportive. 'Yes you've got to do it Mum, it's really good, it's great, we can follow you, we'll be fine,'" she said.
They track their mother's journey on the Internet and pass messages back and forth through the survey command center. All four children will celebrate birthdays while their mother is gone, but parties are planned on her return.
Daniels says despite the arduous conditions of each trek, the constant cold and the deprivations of life on the ice, when it is time to go she feels a certain reluctance to leave.
"I become really quite sad, because I think I may never come back, I may never see this place again. I've been amongst it and been part of it so long it's like leaving a member of the family," she said.
"And then after half an hour or so I suddenly switch and think, 'wow I'm going to see my family again.' And I forget about the ice. You let it go and then you're hugely excited about seeing your family and getting back home."
The explorer's Arctic trip is part of a larger expedition by the Arctic Catlin Survey. Elsewhere in the Arctic a team of scientists will be conducting their own research on ocean acidification until the end of April.