Martin Strel devours the world's most dangerous rivers, only stopping for red wine at lunch. The Slovene may not look an athlete, but he is the greatest long-distance swimmer the world has known, writes Charles Sprawson

"On the morning of February 1st, Strel walked into the water. He would swim for 66 days, 3,278 miles, across Peru and Brazil, with a short break each day for lunch and some red wine, and a few hours' sleep on the boat at night. Every day he would swim twice the length of the Channel.

LONG-DISTANCE swimming has been an epic pursuit since the days of the ancient Greeks, whose mythical romancer Leander swam the Hellespont nightly to be with his lover, Hero. When swimming first became popular as a sport in Britain, in the 19th century, it was endurance feats, not speed, that captured the public imagination.

Hence the fame of Captain Matthew Webb, the merchant seaman who was the first to succeeded in swimming the Channel, in August 1875. Many believed it to be physically impossible, but Webb managed on his second attempt, with a "slow, methodical but perfect breaststroke", not to mention brandy and beef tea.

But Webb struggled to make swimming success pay and 10,000 people watched him meet his end in a tragic attempt to swim across the river below the Niagara Falls in 1873.

In 1926, American Gertrude Ederle was the first woman to swim the Channel, in 14-and-a-half hours. Californian Florence Chadwick broke Ederle's cross-Channel record in 1950. Four years later Chadwick raced Marilyn Bell, of Toronto, across Lake Ontario. In 15-foot waves, a vomiting Chadwick withdrew, but Bell completed the 32-mile swim in 21 hours.

Since the 1970s, another American, Lynne Cox, has dominated open-water swimming. In 1987 she crossed the Bering Strait between Alaska and the then Soviet Union and, even in the coldest waters, wears no wetsuit: in 2002, aged 45, she swam for 1.2 miles and 25 minutes in the Antarctic Ocean.

But Britain, too, can lay claim to one of the best cold-water swimmers in the world: Lewis Gordon Pugh, born in Plymouth, has conquered some of Earth's most hostile waters, including in 2007 becoming the first person to swim at the North Pole, in temperatures as low as -1.7C.

WHEN I first read about Martin Strel swimming the Amazon in February 2007, his predicament took me back to a report years ago of an elephant spotted some 50 miles out to sea in the Bay of Bengal. It had lost all sense of direction and was swimming further and further out. As a passing ship altered course and drew near, the crew noticed blood in the water and realised the elephant was surrounded by a swarm of sharks. It could offer no resistance and swam on desperately, nor could the sailors save it, so they stopped the engines and could only look on in helpless silence as the poor animal was torn apart and devoured in the waves.

There are sharks in the Amazon, too. It is home to the bull shark, widely believed to have killed more humans than any other shark species. But this is only one of a grotesque variety of deadly fish found there, some even more lethal than the piranha, described by ichthyologist and author Paul Budker as "the most aggressive and savage of all aquatic animals". But most feared is the candiru, a vicious little fish that penetrates any orifice available then locks itself into your entrails with a spike and feeds off your blood. Only surgery will remove it.

And there are stingrays and anacondas lurking in the shallows, and the muscular undulations of the electric eel that can stun and drown a man in deeper water. Crocodiles cruise for prey, dark specks on the surface. Long, poisonous snakes can appear from nowhere, while giant catfish have been known to swallow dogs and children. No natives swim there.

All the latent violence and mystery of the Amazon are summed up by an incident that occurred when a photographer I know accompanied the band the White Stripes to Manaus in north-west Brazil. As they walked along the river bank one evening, a fisherman waved them into his house, which floated on logs like all the houses there to guard against flooding. After paying him a fee, they were led through the interior to a veranda beyond, a series of logs tied together, with a railing poised over a metal pen. It was as wide as the house itself, and almost totally submerged in the water below.

As they looked down, they saw nothing at first. Then the fisherman tossed in a bucketful of fish and meat. Soon they could make out a pair of green eyes and a mouth opening wide to swallow down the morsels. The monster's face remained deep in the water, but a series of black muscular coils rose above the surface and endlessly twisted round each other, with a noise and violence that reverberated through the water. The logs below their feet started moving and turning around, so they had to cling on hard to the railing. Then it slowly disappeared. It could have been a python or an eel or a fish, nothing was clear. No one living locally had seen anything like it.

"It's still growing," murmured the fisherman in broken English, then he added: "No one knows what lies at the bottom of the Amazon."

So I thought of the swimmer there in this congested waterway, alone and suspended flat on the surface, a vast hunk of meat surrounded by all these voracious predators, attracting their attention with the splash of his mechanical crawl, in water so black his hand, stretched out, became invisible, and I wondered what chance there was of his ever making it.

He did survive, and I fly out at the end of October to Ljubljana, the capital of Slovenia.

I had arranged to meet Strel and his son, Borut, at 10am the next morning by the entrance to my hotel. Borut is more fluent in English and has interrupted his course in computers to manage his father's affairs.

At 10am there are three people waiting by the front door - me and a middle-aged muscular man in a tracksuit, with a proud warrior's face and the profile of Geronimo, and quite possibly his athletic, computer-minded son. Eventually, after 20 minutes, I ask the younger man why they are there. "To meet someone from the Observer," he replies, so we shake hands and retire to the bar for a reassuring drink.

We then drive into the countryside, to where the swimmer has always lived, about an hour from the capital. We search for the pond where he learned to swim at five. After many inquiries, we find the pond tucked away in someone's backyard, its surface covered with watercress, small and square with granite walls, hollowed out at one end where the water trickles into a stream. In Strel's childhood it lay in the corner of a field. Cattle came down to drink there and women washed their clothes.

The owner of the house, an academic, rushes down with his new book on the history of the region, with a large photograph of Strel. We are offered wine and cakes. He is a hero to everyone we meet, wherever we go.

Strel seems to be able to drive at any speed, and park wherever he likes. The police always recognise him, then smile and wave him on. Slovenia is a new nation. It gained independence from the old Yugoslavia in 1991. Its population is only two million. Presidents come and go, but it is Strel's performances as an athlete, the embodiment of strength and resolve, that have been embraced as a symbol of hope by so many there.

We drive across to his house, shaped like a Swiss chalet, with a pointed roof and a timbered balcony on the upper floor overlooking long views of the rolling, wooded hills. Up to the second World War his forebears - hunters, farmers, lawyers, musicians - owned as far as we can see and beyond, but the nationalisation of land by Marshal Tito, who led Yugoslavia from 1943 until his death in 1980, forced them to forfeit much of it. Strel still owns many acres of forest around him, crammed with wild boar and deer.

When recovering from his river swims, he retires here to play guitar and shoot arrows with a long bow at a distant target - at least too distant for me. He makes his own wine, a refreshing light red, and much of the ground floor is taken up with that. Almost all the rest is devoted to a sauna. He guards his independence. His wife lives in a flat in a village nearby.

He shows me his trophies, piled high in cardboard boxes behind a locked door. With the awkward pride of Jay Gatsby showing off his shirts, he brings some out. He does not seem to know their origins and has to peer hard at each inscription to find out. Among them are a blue bowl from Italy, medallions from China with pictures of Mao swimming in the Yangtse, and the gold keys to various American cities he passed through when he swam 2,360 miles down the Mississippi in 2002 - St Louis, Baton Rouge, New Orleans.

WE SWIM IN a thermal pool nearby, below a hotel and across a series of long lawns criss-crossed by streams that are full of turtles and translucent fish. The pool is made out of planks of wood and has been here since the 18th century, where a warm spring gushes out of the ground, steam rises up from the surface into the cold autumnal air. When in full training Strel swims 6,000 lengths here at five in the morning, then again in the late afternoon.

We move on to the Adriatic coast, not far from the border with Italy - Trieste is only 10 miles away. There is a sandy beach surrounded by hills, with cypresses and pines on the skyline. We step into the freezing water.

Strel is about six feet tall, with a heavy, handsome head supported on a wrestler's neck and a vast torso coated with fat and muscle, which continues down through his legs, though he hardly uses them in the crawl. All his pace and power derive from his back and arms, which are relaxed in the air, with the fingers spread apart, then bend below the surface. As he ploughs through the water, his hips twist as they move, like the undulation of an electric eel. It is not a style that takes one's breath away, but anyone who dived off the bank on his river swims - and there were many, mostly local reporters - and then tried to keep up with him for two or three miles, soon dropped out.

About a mile from his house lie the clear waters of the river Mirna, where he swam as a boy. The dam Strel constructed with blocks of granite, to create a swimming pool, is still there.

When Strel was about 10, a troop of Yugoslav soldiers walked by and, seduced by the pool, challenged each other to a race. Whoever won would be awarded a crate of beer. Strel, at half their age, asked if he could join in, and walked off with the prize.

His original ambition was to become a gymnast, but he grew too big. His coach advised him to concentrate on swimming. "You're a born swimmer," he told him. After he left school he moved to Ljubljana, lived alone, tried his hand at a variety of jobs - bricklayer, garage mechanic - and as a form of release gambled and taught guitar at the local music academy. But he never forgot the advice of his coach. He swam in the local pools, which he hated, then branched out into the rivers around him.

Eventually in 1978, at 24, Strel turned professional and became part of an elite group of marathon swimmers who were invited to compete for prize-money around the world, in races between Capri and Naples, along the Suez Canal, in the rivers of South America and the lakes of the North (he much prefers to swim in fresh water). He was now earning a living as a swimmer and locked into an arduous sequence of engagements he could not afford to turn down.

MARATHON SWIMMERS are a breed apart. Above all, they look different. When compared to the long, sleek, elastic, streamlined bodies of Olympic swimmers, they barely look like athletes at all. Photographs reveal their grizzled faces, their stocky frames and stubby legs, their pendulous breasts and protruding stomachs. Bison rather than gazelles. Some of the best look middle-aged, and often are. The Egyptian Abdellatief Abouheif, the most admired of open-water racers, was still winning in his 40s. Strel swam the Amazon at 52.

And their mentality is different. Their solitary swimming, the long hours spent semi-submerged, induce a lonely, meditative state of mind. Much of their training takes place inside their heads, immersed as they are in a continuous dream of a world underwater. So intense and concentrated are their conditions that they become prey to delusions and neuroses beyond the experience of other athletes.

A psychiatrist reported that after hours of conversation with 14 swimmers who had been champions at underwater swimming, it was clear 12 of them were under severe emotional stress when they won the championship. The remaining two were prone to emotional excess, but took precautions against it. Their tension was typically caused by either the break-up of a marriage, the loss of a job, or sexual maladjustment. They could work it off in training, but in the days before a big swim they relaxed and the neuroses returned. Some were effervescent, while others remained moody and distant, haunted by their thoughts and fears.

Many suffer from self-delusion. Take, for instance, Britt Sullivan, a cheerful, buxom Nebraskan who announced one day she would swim the Atlantic. She hardly trained, and lived on a diet of cigarettes and beer. She set off from the sands of Brooklyn's Coney Island beach in a blaze of publicity. After three days and about 30 miles, she failed to catch up with her escort boat and was never seen again. Fantasies are common. The Argentine Antonio Albertondo, after 43 hours in the Channel (in 1961 he became the first to swim the double from England to France and back), shouted out to his crew that he saw posts, dogs and other strange objects around him.

The conditions they face are extreme. You read of swimmers battling through waves for 30 hours, then ending up in the grip of currents that force them into industrial harbours, streaked with oil, with the ebbing and flooding tides depositing tons of sewage into the water. They have to swim through sharks, jellyfish, oil slicks, bilge from boats.

Then they emerge from the water with their faces enlarged and transformed into some semblance of fungus, with swollen cheeks and lips and tongue.

Some swim for 25 miles, then collapse in the last 30 yards. Many are lifted out unconscious. In 1959 a 46-year-old Greek, Jason Zirganos, attempted to swim the notorious 23-mile North Channel between Northern Ireland and Scotland. Within three miles of the coast he lost consciousness.

He was hauled up into the boat. A doctor cut open his chest and observed his heart fluttering. In five minutes he was dead.

Martin Strel is not one to be affected by emotional stress, or fantasies and hallucinations. He became tired of waiting around in hotel rooms in distant parts of the world, with the same weary faces around him, the same nervous jokes, the tensions of the neurotic, the taciturnity of the tense. A loner, he preferred to do things his own way.

He resists being part of a crowd - he had deserted from the army 37 times. So he broke away and embarked on the series of solo swims that have made him so remarkable.

He swam the Channel in 1997, then became the first man to swim across the Mediterranean, from Tunisia to Italy, the same year. Seven men had already died in the attempt, either drowned by the currents or eaten by sharks. I asked him whether, when he plotted his course and noticed dark patches of blue on the map, denoting great depths, he had been affected at all by any sense of dread, a certain foreboding. He seemed quite startled by this question - every swimmer is — but he just protested the submarine world never preyed on his mind.

Then, between 2000 and 2004, he swam the Danube, the Mississippi, the Parana river in Argentina and the deep canyons of the Yangtse, each successive swim a world record. He also broke the record for the longest continuous swim, non-stop, 313 miles in the Danube, though he slept occasionally while still revolving his arms. But these swims, fantastic though they were, now seem mere preludes to his attempt on the largest, the most dangerous river in the world.

Strel arrived at Atalya in Peru, where the Amazon begins, towards the end of January 2007. No doubt, immediately he got there, he searched out the river, then stooped down, raised the water to his lips and tested its quality - a ritual he performs whenever he comes to a new town. He swam every day, to get used to the water. He watched fishermen bring up in their nets strange, hideous fish he had never seen before and shuddered. After months of preparation he was now ready to start his journey to the Atlantic. His escort boat was stocked with food and every type of medicine.

THE SAILORS WERE standing by, and the cooks, doctors and riflemen armed with automatics. An American film crew was also on board.

On the morning of February 1st, Strel walked into the water. He would swim for 66 days, 3,278 miles, across Peru and Brazil, with a short break each day for lunch and some red wine, and a few hours' sleep on the boat at night. Every day he would swim twice the length of the Channel. He was wearing a wet suit, principally to keep out the candiru, the "vampire of the Amazon", whose intrusions he dreaded. But it would offer no protection from the jaws of the bull shark or piranha.

He started in a tropical rainstorm, which did not affect him as he loves swimming in the rain. Besides, it would be to his advantage, as now the current would flow much faster. He set out with a man in a kayak on each side of him. They made for the middle of the channel, about 100 feet deep, where the water flowed more powerfully than at the edge. He also wanted to avoid the stingrays and crocodiles that favoured the shallows. Piranha are liable to appear anywhere, but they like to congregate in deep, dark holes near the banks of the river, beneath overhanging boughs. It was still water he had to steer clear of, where vague menacing forms flit over the bottom.

Strel told me he was always calling out to the fish, insisting he was their friend. Perhaps his sympathy with water extended to the fish in it. I had watched him soothing and playing with a savage hunting dog that, because of its ferocity, was kept chained to a wall. Maybe he did possess some kind of mysterious affinity with animals and fish. Porpoises and dolphins swam around him for most of the way. The natives called him "arapaima man", because of his resemblance, in his black wetsuit, to the black-scaled fish found only in the Amazon, which, at up to 15 feet long, is the largest freshwater fish in the world.

Strel often felt huge shapes brush against his skin, but would not see what they were. He tried not to look down, he told me, and diverted his imagination from the horrors below with memories of his family - he also has a daughter, Nina - and their life together. Those in the boat could see him mouthing words in the water and often bursting into laughter. It made me think of those lines from Byron's Childe Harold on another son of the Balkans, the gladiator in the arena who in his last moments looks back to where "his rude hut by the Danube lay - there were his children all at play", and their Dacian mother, while he lies dying, "butchered to make a Roman holiday".

He wasn't butchered, though it was his son, there as an organiser, who was the only casualty, stung by a stingray as he stood on the decomposing leaves that litter the bottom. It was insects who proved more horrifying than anything in the water. The flooding was the worst in the Amazon for 100 years. The swollen waters had caused the banks to cave in, and much of the gloomy, tangled forest was now part of the river. Tarantulas and scorpions dropped off trees and floated down on leaves. He could not see them coming, as the current was with him. Larvae burrowed under his skin.

Wasp stings made his head hum for days. Great crawling millipedes hurtled along on logs, as did muniri, the most primitive of ants, which squeak like a mouse and are known to kill dogs. Large black birds flew down to peck at his face. He had to wear a pillowcase over his head, with slits for the eyes and mouth, because he had second-degree burns from the sun.

If the current made him veer to the side, he had to avoid the spikes of palms that lacerate the skin. Chutes formed from narrow channels of heavy water between islands. They could have impaled him on trees whose branches had been torn away, or entangled him in the vegetation. Vast trees were ripped out of the ground and floated past him downstream. He dreaded the roar of the cataracts and the sight of jagged tears in the water, which showed where rocks lay just below the surface.

Around every corner they expected to meet river pirates, who prowl the Amazon in hovercraft that can outpace any patrol boat. The pirates would telephone them most nights, to threaten them and try to break their nerve, but they never materialised. After reaching the official finishing line, Strel decided to swim six miles further, to the port of Belém. To catch the current, he would have to swim at night, the most dangerous time to be in the water.

The psychological effects of darkness are terrible. The imagination goes haywire. The lights on the boat attract every type of fish and there are more fish in an estuary than anywhere else, a mixture of freshwater and seawater, and particularly sharks. He reached Belém before dawn.

A documentary of his exploits, Big River Man, premieres at the Sundance Film Festival this month before opening in Britain in spring. "Martin will become a big star once the film is released!" promises Borut, with a wide smile.

For more info on Strel, visit amazonswim.com. Charles Sprawson is the author of Haunts of the Black Masseur and has swum the Hellespont

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