Iquitos, Peru, population 360,000, bills itself as the largest city in the world that cannot be reached by road. The capital of the Peruvian Amazon, it is an island in a vast ocean of jungle, a seaport 2,000 miles from the sea, linked to the outside world only by air or by the roiling waters of the Amazon River. At the height of the rubber trade in the late 19th century, Iquitos was one of the richest cities in South America, a boomtown that could afford to ship in a prefab mansion designed by Gustave Eiffel or ship out dirty linens to be laundered in Paris. Today it is a filthy, crumbling frontier town, choked with motokars, three-wheeled taxis that turn the dusty streets into a buzzing and honking chaos. Iquitos is also a launching point for exploration of the 2-million-square-mile rain forest that spreads across the Amazon basin, home to a tenth of the world's known species, several of which I can see as I walk along the waterfront, where hawkers sell stuffed piranhas, mounted butterflies the size of paperbacks, and 12-foot anaconda skins unrolled with a theatrical flourish. But I have no time to barter for souvenirs. It's the rainy season, and black-bellied thunderheads are piling up on the horizon as the pressure drops in the soupy tropical air. I am hurrying to the port to catch a boat heading down-river, through the vast unsettled territory that lies between Iquitos and Peru's frontier with Brazil and Colombia.

If all goes according to plan, somewhere on the banks of the mile-wide river I will rendezvous with a 33-year-old former British Army captain named Ed Stafford. But Stafford has warned me that in the Amazon things rarely go according to plan. He should know: Since April 2008 he has been on an expedition to be the first person in history to travel the entire 4,000-mile length of the Amazon River on foot, through the heart of the largest jungle on Earth. He's attempting to walk every step of the river's route from source to sea, wherever it is possible to walk. There are also several hundred tributaries he will need to cross using an inflatable raft he carries with him, and he must traverse three countries and the territories of dozens of indigenous tribes. In his expedition blog, Stafford writes: "Walking from the source to the sea is one of the last great feats of exploration."

We live in an age of diminishing firsts, so those wishing to find fame or notoriety through adventure are forced into increasingly baroque categories: summiting Everest on prosthetic legs, or climbing Kilimanjaro on rollerblades. The Amazon has been run several times by kayaking expeditions, and a Slovenian named Martin Strel has even swum most of its length, but nobody has ever crossed it on foot. When I first read about Stafford's mission, I immediately wondered what made Stafford believe he could actually make it.

Perhaps more than any other landscape, the Amazon jungle is steeped in myth and mystery, looming over the human imagination as a symbol of both untamed wilderness and environmental vulnerability. The mind shudders at its enormity. The river that begins as a trickle of glacial meltwater at 20,000 feet in the Andes discharges 32 million gallons a second. Twenty percent of all freshwater flowing into the world's oceans passes through its mouth, which gapes 150 miles wide. For five centuries the river has been the obsession (and undoing) of countless outsiders, from the lunatic conquistador Lope de Aguirre to the vanished 1920s explorer Percy Fawcett. The lore of Amazon exploration is filled with starvation, madness, disease, and murder.

I understood the region's undeniable allure, but I was still curious why anyone would subject himself to two years crossing a landscape largely populated by anacondas, jaguars, vampire bats, pit vipers, scorpions, wasps, army ants, electric eels, piranhas, drug smugglers, hostile tribes, dengue and yellow fever, malaria, 15-foot black caimans, and 18-inch leeches. Not to mention the candiru, a pin-size catfish that has the ability to swim up a stream of urine and lodge itself irretrievably in the urethra. There are almost no roads along Stafford's route, and since the most common way of traveling is by boat, even trails are scarce. So to try to understand whatever impulse inspires Stafford onward, I arranged to join him for a few weeks of his journey. But first I had to find him.

At the Iquitos docks I board a rusting double-decker ferry, whose every inch of deck space is strung with hammocks. I am the only gringo aboard, and everything I do meets with stares. The sky is flame-orange; cumulonimbus clouds boil over the forest as the boat noses out into the current. The water is the color of cappuccino, and putting green-size mats of floating vegetation drift along in it. Dugouts with outboards hug the banks and beat their way upstream against the flow. The ferry steams along swiftly with the current, the river lashing in broad meanders between the un-broken walls of jungle.

As darkness falls and most of the passengers climb into their hammocks, I stand by the wheelhouse, watching as the pilot navigates around huge floating logs. After several hours of staring into the blackness, a tiny cluster of lights appears on the far bank. As we get closer, I see two men, and the taller one, wearing a baseball cap, waves to me. The big boat grounds itself against the muddy bank, and I jump down, the only passenger disembarking at this stop. The two men approach. I can't resist: "Mr. Stafford, I presume?"

"Have a beer," he replies, laughing.

Ed Stafford stands about 6 feet tall, wearing flip-flops, tattered cargo pants, and a filthy T-shirt, and walks with the bouncy gait of someone who has just set down a heavy load. He has two weeks' growth of beard, an easy laugh, and dark glimmering eyes. He introduces me to his ex- pedition partner, Gadiel Sanchez Rivera, a 28-year-old Peruvian nicknamed Cho. It is 3 am, and percussive cumbia music blasts from the town's only bar, where we drink beer as Stafford fills me in on his story so far. In more than 10 months of walking he has faced poisonous snakes, navigated perilous footpaths above Class V rapids, and had his life threatened by angry tribesmen. He has traveled about 2,000 miles so far and is almost halfway to the Atlantic. "Unfortunately, this has been the easy half," he says.

Inauspiciously expelled from his posh British private school at age 17 for chopping down a tree planted by the queen, Stafford spent four years in the British Army, making the rank of captain. He was once a competitive rugby player and has a false incisor in place of the one he lost on the cricket field. He organized security logistics for the UN during Afghanistan's 2004 elections and led an expedition for a BBC nature documentary in Guyana. He wanted a life of adventure, like that of his hero Sir Ranulph Fiennes, so he and a colleague named Luke Collyer brainstormed possible expeditions they could undertake. Stafford's jungle experience in Belize and Borneo – expeditions he led to raise money for charity – gave him the idea of walking the length of the Amazon. To their surprise, it had never been done.

The expedition began on Peru's Pacific coast in April 2008, with Collyer. The pair hiked up the Colca Canyon and into the Andes, traversing several of the possible sources of the Amazon to cover all their bases. They crossed the mountains with pack burros, and from 18,000 feet began their long descent into the Amazon basin.

As Stafford sees it, there was an imbalance from the beginning. During the months of planning, securing sponsors, permits, and equipment, Collyer was busy with work, so Stafford handled most of the logistics himself. When Collyer arrived in Peru he was out of shape, didn't speak a word of Spanish, and had gotten engaged a few weeks before. "He was totally unprepared for what we were about to do,"Stafford tells me. "And that became more and more apparent as we went on. His heart just wasn't in it."

Three months in, Collyer placed a supply order that contained just one MP3 player. Stafford got angry and asked why Collyer hadn't also gotten one for him. Collyer announced that the player was for Stafford and that he was quitting the expedition. The breakdown had been a long time coming. "The MP3 player was just the final straw," says Stafford. "He claimed he was leaving because our friendship was more important to him than the expedition. For me the expedition is more important than anything." The two men haven't spoken in five months.

When I later contacted Collyer for his side of the story, he e-mailed a polite "no comment": "A lot of time has passed and I've removed myself from anything to do with the expedition," he wrote. "And I'm happy to keep it that way."

Stafford continued on alone, walking with a succession of local guides. Then, in August 2008, he met Cho in the town of Satipo, Peru. Cho had worked for some time as a forester, hiking deep into the jungle to find large specimens of the most desirable timber hardwoods: mahogany, cedar, tornillo. He had initially agreed to walk with Stafford for five days. The two didn't get along at first, but Cho grew enthusiastic about the mission and proved a tireless and loyal companion, and so Stafford brought him on as a paid partner for the remainder of the expedition.

"He's got balls of steel, and he's as keen as I am to complete this expedition," says Stafford. "He's taken the whole thing on as a sort of personal challenge as much as I have. And to find someone like that has been a real key. You just can't do something like this alone." Stafford now has someone to share the weight of food and gear and help bridge the language gap, but Cho's greatest value is psychological: the sheer relief of having someone to watch your back. They have been walking together for seven months now, and Cho has committed to sticking with Stafford until they reach the Atlantic, however long that takes.

Which may be a very long time. Stafford originally planned to travel about 10 miles a day, which he soon realized was "vastly overoptimistic." At that rate he would have reached the Colombia-Brazil border by Christmas. But it was already February, and Colombia is more than 100 miles east of us. The rainy season is in full swing, and the forests alongside the main channel have begun to flood. Stafford and Cho have gotten a taste of that in the last two weeks, crossing the wide delta where the Rio Napo joins the Amazon. "The forests were completely flooded, waist high, sometimes head high," Stafford tells me. "We were scrambling over tree trunks under the water. There's a species of palm here where the entire trunk is covered with three-inch spikes. They were like needles driving straight into our knees."

In the morning the children of the village sit in a hut to watch a badly dubbed version of Jean-Claude van Damme's Kickboxer, and then spill out into the intense sunlight to practice their new moves. They watch raptly as Stafford, Cho, and I organize our gear. My frame pack is stuffed with a waterproof canoe bag; within that are smaller dry bags and Ziplocs, a system that keeps things dry while making them impossible to find. Stafford's bag, a battered 100-liter monster that weighs in at 75 pounds, contains everything he needs to be a self-documenting one-man 21st century expedition. "My kit would be a lot lighter if I wasn't trying to blog this whole thing," he says, as he double- and triple- bags the sensitive electronics that are his only link to the outside world.

Part of Stafford's mission statement is to document the customs and perceptions of the tribes he encounters and the environmental issues facing the region, as well as raise $200,000 for a host of charities. Yet he is the first to admit that he is doing this mainly because it has never been done, and because he wants to have an extraordinary life and support himself with adventures. There's something anachronistic about the project, a "because it's there" attitude that could be criticized as a risky ego trip. I ask Stafford if he feels as if he belongs in an earlier era, perhaps that of Captain Cook or Admiral Byrd. "I feel like I was born at exactly the right time," he says. "I don't think a middle-class individual would have been able to do this sort of thing before." And he admits to an "element of pride about the whole thing," adding, "If anyone has got a problem with it, they should come try it themselves."

Stafford has acquired a full set of 40-year-old National Geographic Institute of Peru 1:100,000 topographical maps, still the most accurate available. In conjunction with his handheld GPS, they actually provide fairly decent route-finding. He shows me his new route, tracing a band of altitude that should – he hopes – help us avoid walking through a swamp and make for faster travel. "One of the odd things about walking the length of the Amazon," he says, "is that you don't actually see the river very much."

For each leg of his walk, Stafford tries to hire a local guide, someone with knowledge of the forest who can help pick the most efficient route. In Oran he has engaged the services of Mario, a 62-year-old farmer and father of 12, who has been hunting these forests for five decades. Mario doesn't stand an inch above 5 feet, and his gear is the minimalist opposite of Stafford's: Everything he needs is stuffed into a small flour sack that he carries by a cloth strap across his forehead. The only other items he has are rubber boots, a machete, and an ancient rusting shotgun, in case he stumbles across dinner.

Shouldering our packs, we turn away from the river and cross a cow field behind the village, the tropical sun crushing down on us. A few 100-foot shade trees have been left standing alone, a sobering indication of the original height of the rain forest's triple canopy. This part of the Amazon, too remote and flood-prone to be easily exploited, still offers glimpses of the devastation wrought elsewhere.

We make our way up a slope, and within minutes we plunge into the tangled green wall that closes off the edge of the forest, leaving the bright world behind. Even at noon on the equator, the jungle is dim, the filtered green sunlight offering little sense of direction or time. The air is cooler, sounds are muffled, and the line of sight is reduced to a dozen yards through the dense understory tangle of creeping vines, lianas, and sprawling root systems. Huge trunks shoot up through the canopy, clung to by vines and strangler figs, giant bromeliads hanging like chandeliers. An astonishing amount of biomass claws upward, trying to bridge the gap between the limitless water of the ground and the limitless sunshine of the forest roof. You can almost hear it growing. The leaf litter on the ground is a foot-deep cushion, and there is no sound but the drone of insects and the distant calls of birds.

Mario leads the way along a barely perceptible path. His machete seems to be an extension of his body, and he parts the jungle with deft ease, using only the tip of the blade to slice thick vines and huge leaves. Stafford has also become adept with the indispensable machete but still relies on brute force to hack his way through obstacles. The diminutive Mario doesn't even sweat and seems to expend almost no effort as the trail parts before him with a flick of his wrist.

Taking up the rear, I'm already soaked through with sweat as we balance our way across mossy logsspanning tea-colored streams and scramble over waist-high buttress roots. Mosquitoes swarm around us, and hordes of stinging ants brush off from overhanging leaves or the trunks of trees. Even the vegetation has evolved with its own aggressive microspecializations. There are spike-covered roots that seem to grow exactly where a handhold is required, vines like rubber bands that wrap around my ankles, and thorny tendrils that snatch the hat right off my head. The worst by far is serrated razor grass, which slices through clothing and skin with the lightest touch. Stafford has been told there are endless stretches of razor grass down river in Brazil, another obstacle to add to the preposterously overfull roster that stands between him and the mouth of the Amazon.

A few times Mario stops dead and points off into the underbrush. I see nothing moving at all. "Pit viper," Stafford tells me. I've researched enough about the variety of horrible deaths on offer in the Amazon to know that a pit viper's hemotoxin causes massive hemorrhaging, bleeding from the eyes and ears, necrosis, then death. "Oh, don't worry," Stafford says cheerfully. "We've got six doses of dry antivenin, enough to last 18 hours, and there's a military rescue helicopter in Iquitos. The worst-case scenario is if you were bit at sunset, because the helicopters can't fly at night. But we'd be able to keep you alive until dawn." Well, then. No worries. The expedition has already come across 10 pit vipers, all but one of which have been quickly dispatched by their guide's machetes. In stark contrast to our own conservation dogma, no local guide would let a poisonous snake escape if he could help it.

The only anaconda they had come across while walking was a beautiful 12-footer that Stafford stopped to film. When he'd finished, his native guide hacked the creature into pieces. "He said it was to feed to his dog," Stafford tells me. "But one of the things I learned early on was that there was no point in trying to impose my Western sensibility on the people who live here. They do what they do to survive. They don't think of animals as having any value except food."

In late afternoon we find a small stream and stop for the night. After months of sleeping out, Stafford and Cho have reduced setting up camp to a science. I have not reached that point and very nearly dismember myself with the machete while trying to clear an area of underbrush. Eventually I get a tent fly strung between two trees as a shelter. This way camp can be made even in a driving rain. The key piece of equipment is the expedition hammock, enclosed by mosquito netting and entered via a Velcro- sealed slit in its bottom.

Cho and Stafford assemble a structure of damp green wood on which to build a fire and support the cook pot, gathering standing deadwood from the forest for fuel. It's a miracle the fire will catch with wood that is drenched daily, but Cho soon has a crackling blaze and puts on a pot of stream water. Dinner is boiled rice with canned tuna bought from a supply store in the last village we'd passed through. "When I first began, I thought there would be much more of a survivalist element, fishing and living off the land," says Stafford. "But as it's happened, we come across villages often enough that we can resupply or pay villagers to cook for us. That's been the most surprising aspect: how much we've had to deal with people. I had imagined it would be emptier, more man versus nature." He realized that the cultural interchange with the people of the Amazon was an integral part of the expedition. He also realized that it was easier to start a fire with a cheap plastic lighter than with flint and steel.

Even though we're coated with DEET the mosquitoes are swarming around us, and as the equatorial night drops fast I crawl into my hammock and close myself in. A symphony of insects performs, multilayered, shockingly loud. Thousands of mosquitoes tap against the netting, probing for an entrance. The cough-like whoops of howler monkeys echo in the distance. Then a low hiss builds and builds, until the temperature drops and the rain opens up like a jet taking off, drowning all other sound, enshrouding the night.

In the morning I put on my still-wet clothes from the previous day. Wrung out is as dry as any of us ever get, and Stafford tells me to look out for foot rot, staph-infected cuts, and all the other bacterial and fungal delights of the Amazon's petri-dish environment. In an uncaffeinated haze I remember that you are supposed to shake your boots out before putting them back on in the jungle. I tap one upside down and a cricket the size of a sparrow clambers out and hops away.

The Amazon has a keen sense of irony. Mention how easy it is to cross a log bridge, and you will do a gainer into a stream; praise the quality of the trail, and it will disappear into a swamp; comment on the fine weather, and a Wagnerian thunderstorm will ensue. Stafford has gotten used to the frequent mishaps and come to see them, afterward, as a kind of comic relief. One afternoon he left the map behind when we stopped for lunch.

Mario, far faster than any of us and more certain of his direction, dashed off to retrieve it and returned at nightfall. Months earlier Stafford dropped his only machete during a river crossing and had to push on to the next village with his bare hands. Self-deprecation seems to be a key to his success so far, and he has the ability to take the expedition seriously and recognize its absurdities at the same time. Plunging up to our necks in a creek crossing, he mutters, "Bloody silly expedition," and soldiers on.

For days we continue slogging along the contours of the chart, different only in scale from the columns of leaf-cutter ants that march alongside us. It is exhausting, dirty work, and I am covered with mud, scratches, and bruises. The prospect of doing this with no clear end date would be daunting. Stafford stops periodically to check our progress with the GPS. Mario looks on politely, though he has no idea how to read a map and the GPS is an impenetrable mystery to him. Stafford defers to Mario's local knowledge but likes to double check against modern technology. "I know with this I'd be able to make it without a local guide," says Stafford, "but it would be much slower, and much more work." The GPS shows that Mario has taken us almost exactly along the planned route.

It's disheartening to see indisputable data on how slow our progress has been. On a good trail, two or three miles an hour is reasonable, but in trackless jungle, scrambling over or under fallen trees, hacking through vines or wading through mud, forward movement can slow to an agonizing crawl. After struggling with the frustration of slow progress for much of the trip, Stafford has finally reached some sort of peace with it. In the slog through the Napo delta, he noticed that in chest-deep water his heavy pack became buoyant, and there was a "bizarre sort of serenity" as he made his way through the silent flooded forest. "For some reason my default mode is military, 'we've got to get there,' " Stafford explains to me. "Cho takes his time walking through water. Suddenly, I wasn't getting frustrated walking only 2.5 kilometers a day. It was tranquil, and I realized it's going to take as long as it takes."

Although Stafford has become accustomed to the physical hardships of the jungle, encounters with tribes remain Stafford's greatest challenge. Many villages speak unique dialects, using only rudimentary Spanish. There is a long history of exploitation of tribes by oil and gold prospectors, and thousands of indigenous and rural Peruvians were murdered during the years of insurgency by the Maoist Shining Path guerrillas. So it's for good reason that many indigenous communities harbor a deep suspicion of outsiders.

One of the most pervasive fears is that white people are pela cara; literally it means "face peeler," but the term has become a myth among many native communities that outsiders will steal their organs. "The last thing you want after an exhausting day of walking is to arrive someplace and have the whole community be scared of you," Stafford says, but he has learned how to stay calm, how to deescalate tensions.

Once, upon entering a village back along the Apurímac, Stafford was immediately confronted by an angry mob of Indians. They poured water on him, shoved dirt in his mouth, and smeared his face with red paint. He was scared but did his best to stay calm. "I just shook hands with their chief, turned around, and walked out of the village," he recalls. Not long after that incident he and Cho were crossing a tributary in the pack rafts. Cho looked over his shoulder and saw that they were being followed by five canoes filled with furious Ashaninka Indians. The men were armed with bows and arrows and shotguns; the women carried machetes. "I was pretty sure we were going to die," says Stafford. Even Cho, normally unflappable, thought they were done for. They were surrounded, and the leaders of the tribe approached them, screaming, blind with rage. Stafford showed them their permits from the regional authorities, but nothing helped. The women seemed ready to hack them to pieces. Finally, speaking slowly and quietly and holding his hands open, he managed to get them to calm down. Andreas and Alfonso, the leaders of the tribe, ended up joining them as guides for six weeks. Stafford was astonished. "The people I was most afraid of on the entire expedition turned out to be the most kind, helpful, and loyal people I've met."

That experience has convinced Stafford that he'll be able to handle whatever situation arises downriver. But Brazil presents even greater risks. When Stafford applied for permits through a fixing agency in Manaus, he initially got no response. "When I finally reached them," he says, "they said they didn't respond because I was going to die. 'It's a suicide mission. The indigenous reserve on the other side of the border in Brazil is the fiercest in the Amazon basin. Colonial Brazilians don't even go there. You're white, don't speak Portuguese, and are wandering around with a video camera.' " All salient points, he thought, but decided he'd "just go in and be very friendly and very calm." He still believes that with the right guides and the right approach, he'll make it. "But I have yet to meet a Brazilian who thinks it's possible," he concedes. "The only people who say 'Yeah, you'll be fine' are your friends back home, who haven't a fucking clue."

While Stafford measures the risks rationally, Cho has a more mystical outlook. A deeply religious Christian, he believes that God is protecting them. Stafford is more fatalistic. "I am either going to make it, or I am going to die trying," he says in a way that is almost cocky, confident that he can manage the risks and come out the other end alive and victorious.

We stay in a Yawa village for a day, where I entertain the children with my Buster Keaton antics, smacking my head on five-foot-high door frames and falling out of hammocks. A boy in a dugout paddles us across the tributary Rio Apicuyu, loaded high with our gear. Drifting, watching toucans and scarlet macaws alight in the trees by the riverbank and huge iridescent blue morph butterflies rising on the breeze, I am struck by the folly of Stafford's "bloody silly expedition." The Yawas paddle up and down the river on dugout canoes, slipping easily with the current wherever they wish. All the cultures in the Amazon make use of the thousands of miles of waterways. Walking the Amazon seems analogous to crossing the Sahara on snowshoes: You could do it, but it's certainly not the way the locals go. There's a reason nobody has ever done this before.

I bring this up to Stafford, and he laughs. "A friend of mine once said, 'I fucking love your expedition because it's pointless.' It's a real British mentality: It's fucking pointless, but we'll do it anyway." Like Livingstone and Scott before him, Stafford has completely bought into the stiff-upper-lip masochistic absurdity of his endeavor, and he's proud of it.

Sore and scraped up after three more days of hacking our way through maze-like jungle, we finally reach the next village. Porvenir is an idyllic scattering of thatch houses on stilts set on a bluff above the Rio Ampicuyu, another small tributary of the Amazon. From here we must temporarily leave the route of the expedition to rendezvous with Pete McBride, a photographer from Colorado. We spend five hours in a dugout canoe, motoring downstream to the ramshackle market town of Pevas, right on the Amazon itself, where we meet up with McBride, resupply our stocks of tuna and ramen noodles, and return to the spot where we'd left off.

This is one of the self-created regulations of the expedition: Whenever Stafford leaves the route, he sets a GPS marker so he can return to the exact spot and pick up where he left off. It's what makes the game of walking the Amazon fun, a stickler of a rule that presents all sorts of logistical challenges. There is, of course, nobody to enforce this except Stafford and Cho, but the idea of cutting corners is unfathomable to them. "I wouldn't bother suffering this much if I were going to cheat on the small things," says Stafford. "If we're going to do this, we're going to do it right."

Mario returns home to his family and village, and for the next leg of the journey we will travel with a guide named Bernobe Sancha, a 38-year-old Ocaina Indian. I wake up in the morning to find Bernobe standing perfectly still, perched on a root above the edge of the river, holding a machete. With a quick flick and a splash, he hacks downward. A fish, its head surgically cut in half, drifts up to the surface of the water. We gut it and split it for breakfast, five ways.

When we walk out through the fields behind Porvenir, I stumble across a well-tended little plot of coca bushes. We are only 50 miles south of the Colombian border, and a huge amount of drug trafficking passes through the region. Peru is the world's second-largest producer of coca, whose leaves are refined into paste before being trekked to drug labs across Colombia's border for further processing. En- counters with nervous traffickers, the vast majority of them poor Peruvians, will be a serious risk as Stafford and Cho approach the "Triangle of Death" at the Peru-Brazil-Colombia frontier. In Pevas I read an account of a Peruvian village that had been burned to the ground in a turf war between rival drug gangs. It was right along Stafford's planned route.

The jungle life is beginning to wear on me after 10 days of trekking. As my willpower flags, my astonishment at Stafford's determination grows. My feet, soaked for 12 hours a day, look cadaverous. I long for water that doesn't taste like an iodized puddle. I am covered with ant bites, and my ankles are embedded with parasitic fleas. At one point Stafford stumbles into a swarm of wasps, and the four of us sprint in a panic back down the trail. Then, while crossing through waist-deep water, McBride looks down and shouts, "What the hell is that thing?" This is not something anybody wants to hear while standing in an Amazonian swamp.

The creature has a huge whiskered head like a catfish, but a bright red mouth and a tail that winds off behind a stump and breaks the water six feet away. It swims slowly toward McBride and then vanishes below the surface in the murk. Bernobe tries to explain in broken Spanish, repeating the word anguila, but none of us knows what it means. Only later do we realize that the thing was an enormous electric eel, which could have generated enough of a shock to knock us all unconscious, facedown in the water.

On the day McBride and I are to leave, we have to make our way to the bank of the Ampicuyu to meet with a boat down toward the Amazon. According to Stafford's GPS, the river is 800 yards away. We strike out toward it, hacking through vines and undergrowth, but after just a few yards there is nothing but flooded forest as far as I can see. The only way to the river is straight ahead. We are knee deep, then waist deep, and then the dry bag in my pack begins to float and the weight is lifted off my aching shoulders. Our footsteps are silent, and we glide around enormous root buttresses in the light-dappled water. The flooded forest is otherworldly, literally: an exact replica of the forest, the sky, and our- selves moving in reflection over the still black water. When it is too deep we load our bags in the pack rafts and push them through the water, swimming in our heavy boots, laughing, spiders and ants on every branch. The water is too murky to even see a hand below the surface – or an electric eel.

The flooded forest is the epitome of all childhood nightmares, and yet I'm not afraid. I now understand the realization Stafford came to while crossing the Napo delta: He learned to let himself float, to feel the tranquillity of the moment he was in. Stafford has a long way to go, perhaps 18 more months, but you can't rush an expedition like this. It will take as long as it takes.