'I've lived very close to death,' says Horn, the deep lines on his face illuminated by a wintry sun that's sprinkling Sydney Harbour with silvery light. 'But a miss is as good as a mile. Either you're dead or alive. I would rather be dead than experience a living death. But I don't have a death wish. The strongest will I have is to survive. I'm not afraid of losing, because I know nature can beat me every time. And losing means I lose my life.'

Horn touches the ends of his fingers as he speaks. The tips are missing, a legacy of his first attempt to walk solo and unassisted to the North Pole. So does it hurt, skiing for just over 60 days - the time it took him to conquer the pole in 2006 - with no sunlight, in temperatures that range from a frigid minus 30 degrees Celsius to an almost unearthly minus 60 degrees?

'It's painful, it's really, really painful,' he says of experiencing 97 degrees below the body's normal core temperature. 'You control the level, which is always in the red. To put it less poetically I'd say [on the ice] I'm always in the s**t. But pain tells me that everything's still there. I'm afraid when I stop feeling pain, when I start feeling comfortable, that's when there's a problem.'

Friends and rivals say Horn is the world's most versatile and courageous solo explorer but for the current phase of his career he is travelling with 144 young people, mostly teenagers. Over the course of four years and 18 stages (Horn calls them 'projects'), eight young sailors at a time will be living on board Pangaea, the largest polar exploration sailboat ever built.

The vessel is travelling to all the continents and close to the North and South poles, assessing the state of the environment as its crew sails and treks through some of the planet's most inhospitable and endangered terrain. Those who want to participate in Horn's Young Explorers Programme must be under the age of 21 and demonstrate that they have strong leadership qualities. Income is irrelevant, although many parents have offered Horn big money to take their offspring on board.

'If you want to go on an expedition you must do a lot of preparation,' says Lee, who had never seen real snow before she ventured to Switzerland, where she became famous for screaming 'Chinese girl needs help' as she dangled from ropes. 'You have to prepare mentally and physically because without that and the right professional equipment you can die at any time. But if you are willing to do the preparation, nothing will stop you.'

The young explorers' expenses are paid for by Officine Panerai, an Italian luxury watch company with a seafaring history. But when Horn came up with the idea for the Young Explorers Programme (YEP), he had no money and no sponsors. Not that a small thing like that would slow him down.

When Horn takes the helm of Pangaea, to steer the 35-metre-long yacht through the tricky currents beneath the Sydney Harbour Bridge, he looks like he was born on a boat, but he did not take up sailing until he was in his 20s. When it comes to sport, his first love was skiing and he fell into that by accident.

'I was born in South Africa, at a time when South Africans did not have many travel options,' says Horn, pausing to order his crew to trim the mainsail. 'We could go to Switzerland and that's where I went, broke and unemployed.'

Horn was living on whatever food he could 'forage' from people's gardens and that, with winter coming on, was not a sustainable plan. He decided to hitchhike to Israel.

'This guy picked me up and I told him where I wanted to go and he said it was impossible,' recalls Horn. 'He told me he was going to Chateau D'Oex and I could go there.'

The driver, who owned a local hostel, dropped Horn in the tiny Swiss village two hours from Geneva at midnight. A few minutes later he returned to offer Horn a bed for the night. A few weeks later the man went on a business trip and asked Horn to look after the hostel. Horn stayed, learned to ski that winter, became an instructor and now has his home and training centre in the village where he first arrived as a penniless stranger.

As Horn reached his mid-20s, his hunger for adventure drove him to ever more risky trips. He decided to conquer the Amazon. Then he was circumnavigating the globe at the Antarctic and heading to the most dangerous peaks in the Himalayas (not Everest, which Horn regards as 'too easy, too obvious'). And it was on the mountains that he had the idea for the Pangaea expedition.

'I was thinking about what I had to do as an explorer, did I have to keep going to the poles and climbing mountains,' he says, back in port and seated in a restaurant with a cold beer. 'I thought that would be selfish, because in a way you become like a stress-seeking animal, just trying to find the next challenge, which was easy because I knew I could always do it alone. But what if I took younger people? Would I be able to cope with that? That would be a real challenge for me, because I never wanted to explore with somebody else.'

Horn also wanted to use the expedition to advance his own theory about what is happening to our planet. He gets irritated by what he calls 'the big global warming bandwagon', which he believes has turned the environment into a political football, and he thinks 'decisions made by politicians are one of the biggest reasons we are in the situation we are in today'.

The United Nations wanted to co-opt the Pangaea project and give Horn's young explorers diplomatic passports but he baulked at the loss of independence. Instead, Horn wants his crew to be free to speak their minds and to use the expedition to identify discrete examples of climate change and work out ways to deal with them. And few, if any, are better placed than Horn to talk about how the poles have been affected by pollution and greenhouse gases.

'When you see the first attacks of grizzly bears on polar bears or grizzly bear tracks on the polar ocean where they did not come before, that's a definite change,' says Horn. 'And when the glacial ice at the South Pole is slipping into the ocean and I see a mother and cub at the North Pole where they shouldn't be, where before it was only lost polar bears or old males, then that's evidence of a quick and severe change in the climate.'

Horn hopes that by cataloguing these profound changes with young explorers and involving them in experiments to monitor future shifts, he can help create a new generation of leaders - he calls them 'change agents' - who will tackle environmental issues in a different way.

First, though, he needed a boat. And that became an expedition in itself.

When Horn began planning for the Pangaea expedition he decided he wanted a technologically advanced yacht with a unpainted brushed aluminium hull that would not damage the environment in any way (the gleaming white paint on yachts and cleaning fluids cause 20 per cent of the pollution in the world's oceans) and he wanted it ready in 12 months. The problem was such a boat would take four years to build and cost far more money than Horn had available.

But men who have beaten frostbite at the North Pole and legions of bugs in the rainforest don't give up easily.

'I went to this guy in Brazil and asked if I could buy 80 tonnes of aluminium,' he says. 'He told me I'd need two million euros [HK$21.9 million]. I said, 'I don't have money but I need the aluminium today.' The guy asked me why and I explained the project. He said he wanted to help me, so he gave me the aluminium, with the condition that if I did not pay him in six months the boat would be his.'

Horn now had another problem. He had been planning to build the boat in Croatia but had no money to transport the huge amount of metal. So instead of going to the airport he went to the slums of Sao Paolo.

'I drove around looking for guys who could work with aluminium, wood and other materials and we began building the boat right there, so they could all walk to work,' says Horn. 'The condition was that they work 24 hours a day, six hours on and six hours off. I organised catering and we recycled all the aluminium scraps to pay for the food.'

Horn had his boat in 13 months, although he had to 'steal' it from under the noses of crooked customs inspectors who were planning to seize it the moment it hit the water to hold it for ransom.

Then came Horn's other moment of good fortune. In 2001, he was in Europe to receive the Laureus World Alternative Sportsman of the Year award, which was sponsored by the Richemont luxury goods group, owned by fellow South African Johann Rupert. The group includes the Panerai brand. After the presentation, Rupert, clearly moved by Horn's exploits, came up to the explorer, took a Panerai watch off his own wrist and handed it over.

He looked Horn in the eye and said, 'From now on this watch brand will be your sponsor and you don't need to worry about money.'

Rupert stuck to his word. When Horn asked Panerai to back Pangaea it did not hesitate.

Horn began the Pangaea expedition from Ushuaia in Argentina, setting sail in November last year. At Hercules Inlet, Antarctica, he was dropped off to begin his solo trek to the South Pole, hauling a sled with 196kg of equipment and supplies. He was in the territory that once swallowed Shackleton's Endeavour and took the lives of Robert Falcon Scott and his four companions. What was it like to complete another near-impossible journey?

'Disappointing,' says Horn, his South African accent becoming more intense, as it often does when he's amused or annoyed. 'When I reached the South Pole I was walking over the landing strip at the [Scott-Amundsen] American base and I didn't notice a red light flashing. This American guy comes out and yells, 'Didn't you see the light? A Hercules was about to land and you've broken the law. We'll sue you.' And I said, 'Who's law?' I was astonished. I had been alone for two months and now some jerk is asking me to stop at a red traffic light at the South Pole.'

The human element has often been the most bothersome aspect of Horn's expeditions and, despite brushes with venomous snakes and lethal frost, it was man that brought him closest to death.

'I was put in front of a death squad in the Congo, when I was walking around the equator,' says Horn, grim-faced. 'They killed the guy next to me but kept me alive. I thought that day I was done.'

Horn says he was rescued from the government forces - who had mistaken him for a rebel fighter - by a policeman shortly after the man kneeling next to him had been shot. The policeman convinced the troops Horn's case was a police matter.

He had spent the previous night waiting for his execution.

'All that night I was asking myself if I had done all the things I really wanted to do,' says Horn. 'And what disappointed me was that after all the challenges I had set for myself, I was so prepared for nature and so unprepared for humans. Nature you can predict and control. With humans you turn around and they stab you in the back.'

Maybe that experience is partly why Horn is determined to use the Pangaea expedition (which will make a stop in Hong Kong in September, en route to India and the fourth YEP project) to teach the Young Explorers lessons about human nature, as well as the natural world.

During the New Zealand project, in the middle of a long hike, Jun Lee fell behind the others. She encountered a man in a truck who offered her a lift back to Pangaea, eight kilometres away. Lee says, for her, it was natural to accept and she was showered and fed by the time the others arrived at the boat feeling tired and sore.

'The others were very angry with Jun,' recalls Horn. 'They felt she had cheated. So we talked it out and I asked Jun to go back to where she was picked up and walk the final [eight kilometres]. I offered to go with her.'

Lee was even more surprised when the other explorers offered to do the walk as well.

'I didn't think what I had done was such a bad sin at first,' she says. 'But I'd ignored the importance of team work. It taught me to be less selfish.'

Lee also learned a great deal about the environment when she was in New Zealand, the most notable fact being that there are places in the world that have clean air to breathe.

'I have had very serious respiratory problems living in Shenzhen,' says Lee. 'But in New Zealand everything was OK. Even my asthma, which many of my friends have in Shenzhen, went away.'

Since returning to Shenzhen, Lee has set up a number of projects in her school to conserve water and save energy. And she has become involved in the school newspaper, to be an advocate for environmental causes. This is what Horn has in mind for what could be his most challenging quest to date, to find a way to halt climate change.

Does Lee, brought up in a polluted mainland city, think Horn has a chance of getting on top of this problem?

'I think that it may be too little too late,' she says. 'On the mainland, it feels like the air is heavy. But everybody who I was with in New Zealand was changed by what they saw and maybe that's the best reason for hope.'

For Horn, that's why he goes on expeditions, not principally to find out more about the planet, but to discover more about himself, to see his spirit evolve.

'I'm quite an emotional person but I don't cry often enough,' says Horn, his muscled shoulders spread wide across a bench on Pangaea. 'I cry in myself. I don't physically cry but I have my heart crying. Sometimes I'm disappointed in myself and frustrated and it's good to just sit down in the tent and cry, just to get rid of the built-up tension. I can't do that so easily when I'm back home. That's why the most difficult time of my life has been each time I've reached the end of an expedition.'

Given the scope of the Pangaea project, to map and try to resolve climate change, it may be some time before he reaches the end of his current quest. And Horn is fine with that. He wants to save the planet and he's prepared to go to the ends of the earth to do it.