It's a six-month expedition in almost constant darkness, in the coldest place on the planet, with no chance of rescue if things go wrong. British explorer Sir Ranulph Fiennes calls it one of the last remaining polar challenges: crossing Antarctica during the region's winter.

Against the backdrop of Cape Town's majestic Table Mountain, Fiennes, 68, and his five-member team left the South African port city on Monday aboard a South African polar vessel, the SA Agulhas, for what they have dubbed "The Coldest Journey."

After reaching the southernmost continent, the expedition will begin its journey via the South Pole on March 21, traversing nearly 4,000 kilometers (2,485 miles) in a place, beautiful and forbidding, where temperatures often dip as low as minus 70 Celsius (minus 94 Fahrenheit).

Or colder. Antarctica has recorded the lowest temperature anywhere on the planet — a shocking minus 89.2 Celsius (minus 128.56 Fahrenheit).

The trip is particularly hazardous because no aircraft can travel inland in the winter due to the darkness and risk that fuel will freeze, meaning there is virtually no chance of a search and rescue operation if disaster strikes.

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Even Fiennes, who has spent a lifetime embracing peril, is circumspect.

"I usually look forward to expeditions, but there is such a big degree of uncertainty with this one that looking forward to it is probably not the exact right word," Fiennes said, according to the website of SABC, South Africa's state broadcaster.

"Some people will say it is irresponsible to go unless you know everything, in which case the Americans would never have gotten to the moon. If humans are going for something new, then unfortunately there are bound to be some gray areas," Fiennes said.

According to Fiennes' website, British authorities had not previously granted permits for winter expeditions in Antarctica because they were seen as too dangerous. Satellite and other communications technology will allow the team to communicate with the outside world and to provide updates on its progress.

Fiennes and his team will have high-tech gear, including battery-operated heating mechanisms in their clothing and special breathing apparatus. They will use modified, 20-ton tractors to transport sledges with mounted living quarters and fuel that is designed not to freeze in the extreme temperatures. They will also have radars that can detect crevasses.

Anton Bowring, the expedition co-leader who is traveling on the polar ship but not joining Fiennes on the ice, said the modifications to clothing and equipment for the polar trip made it comparable with preparations for a flight into space.

"Psychologically, the conditions are similar," he told South Africa's Sunday Times. "Once they set off and the winter sets in, they are on their own. You can't get an evacuation if someone gets appendicitis or frost bite. There is nothing you can do about it."

Spencer Smirl, a Canadian mechanic who will travel all the way with Fiennes, surfed the waves and got a sunburn during his break in Cape Town. In a blog posting Sunday, he described his excitement at facing the "deep freeze" in the weeks ahead.

"As much as I can anticipate I will be very seasick, I can't wait to see what the extreme violence the southern ocean can provide," he wrote. "It will be a very new experience for me. One that I don't think I will ever forget."

Expedition organizers plan to raise $10 million for a charity that seeks to prevent blindness. Team members also hope to conduct research aimed at understanding the effect of climate change on the poles. Their project has taken years to plan.

If all goes well and they complete their journey as scheduled around Sept. 21, they will still have to camp out and wait until January 2014 to head back to Britain. Only then, when temperatures are comparatively warmer, will their polar vessel be able to reach them.

Fiennes has compiled a long list of achievements over the decades, many involving the Antarctic and Arctic regions. He became the oldest Briton to summit Mount Everest in 2009. He is missing parts of his fingers on his left hand because of frostbite that he suffered on a North Pole expedition more than a decade ago.

According to the "Coldest Journey" website, expedition patron Prince Charles wished Fiennes and his team "God speed and every possible good fortune in this wonderful, dotty adventure."