Despite nature's unpredictability, animals are able to achieve incredible feats. Join us next on Life on Fire as Alaska's sockeye salmon journey into the very heart of a volcano. PEOPLE OF THE VOLCANOES (narrator) Every river carries its own unique scent down to the ocean. Salmon gather to catch the scent and follow the trail upstream to their birthplace. In Alaska, the fresh water that feeds the rivers is snowmelt from North America's highest mountains. But under many of these ice-covered peaks, a furnace burns fiercely. For these are also North America's most active volcanoes. Time and again over the centuries, they have erupted, turning the crystal-clear river waters black and poisonous. Scientists have only just begun to piece together what might have happened nearly 2,000 years ago, when one population of salmon faced the death of their natal river and were forced back to the open ocean on an extraordinary adventure,... ..leaving behind an amazing legacy. THE SURPRISE SALMON (narrator) Young sockeye salmon feed and grow in a vast stretch of the Pacific Ocean, where warm waters from the tropics mix with cold Arctic currents, creating rich plankton blooms. But when they're five years old, instinct draws them back to the Alaskan coast. They gather in large shoals, each salmon seeking the scent of its native river. These silvery fish have grown fat and ripe on plankton. But now they stop feeding, their one goal: to swim to their birthplace to lay their eggs, or spawn. But the ocean is not as clear as it should be. The water carries with it a faint scent of sulfur. The mountain which witnessed the salmon's birth is on fire. The remote Alaskan peninsula lies on the Ring of Fire. The movement of the ocean plates traps water under the earth. Heated under pressure, the water turns into gas and escapes through any crack it can find. This area is still highly volatile today. The volcanic activity, dating techniques and genetics make it possible to recreate the story of one population of salmon, as their world began to fall apart nearly 2,000 years ago. The salmon are confused. They can smell the sulfur dissolved in the water. But still more powerful are the scent molecules, the unique chemical signature of their native river, and this is irresistible. Upstream, a thick sludge of black ash is heading their way. The summer sun usually melts the white snow, causing the rivers to run fast and clear. Now heat from the ash and sun combine to burn new channels through the glacier itself, pouring deadly toxins down its flanks. Unwittingly, the river gathers up all the dregs and poisons and carries them towards the fish. By now, the whole valley stinks of death. Too late for some. Luckily, downstream, a few latecomers heed the warning signs and slowly turn back. They have no choice but to retrace their journey back to the open ocean. It's not so much that they sense danger, but that the freshwater signal drawing them in has been switched off. With many of their number dead, these are now lost salmon with no sense of direction. Yet the urge to breed is still powerful. Incredibly, each individual salmon begins a lonely quest, hugging the coast, instinct driving them to begin again and seek out the scent of fresh water. Sockeye salmon have remarkable tools to help them in their search. Their formidable eyesight allows them to see both above and below the waterline, right way up or upside down. The volcanic ash has stirred up the ocean, stimulating the growth of plankton and encouraging clouds of stinging jellyfish to bloom. It's like swimming through a minefield. But the highly sensitive salmon can actually smell these jellyfish and so avoid their deadly tentacles. The sockeyes have to keep moving. It's only in the short summer season when the sun melts the glaciers that the rivers run deep enough for the salmon to swim upstream. This sockeye has swum all the way to a sweet-smelling river only to find it impassable. The nickname for salmon is "jumpers", but sometimes even they are defeated. They never stop to feed or sleep. They're programmed to find fresh water or die trying. Salmon sharks also have a superior sense of smell and they use it to hunt down their namesake. The oily scent of the salmon's skin gives them away and the sharks lock in. Salmon sharks have the warmest bodies of all fish. They can retain the heat created by their muscles, making them even quicker off the mark. The gulls, always bad news for salmon, finish the feast. The survivors just keep moving on. Sockeyes have incredible stamina. In their lifetime, they can travel over 6,000 miles. Once again, the scent of fresh water lures the salmon close to the shore. But this mountain, named Augustine by Captain Cook, is another active volcano. Rain clouds condense on its summit. But its internal furnace prevents ice from forming or snow from settling. On Augustine's steep slopes there are no quiet backwaters where sockeyes can breed and no river valleys up which they can travel. Fresh water trickles into the ocean, but it's of no use to the salmon. Because all these travelers are after the same thing, their numbers are beginning to swell. Like moths to a flame, salmon are attracted by fresh water and will keep on swimming, tracking the chemical gradient towards its source. Several miles on, and once again the salmon's nemesis is waiting. (gulls cry) At the river mouth, a staircase, easy enough for an acrobatic sockeye to leap. And the tantalizing scent of fresh water gushing towards them. The salmon are intoxicated. There are so many salmon because the travelers have been joined by another population of sockeye, whose river this is. It's a good omen. If there are other salmon, there must be places to spawn. The two populations look alike. Only their genes tell them apart. But they run the gauntlet as one. For those that make it past the gulls, the trap slowly closes in. (growling) The tide is already turning against the salmon. These grizzlies need to double their body weight during the short Arctic summer to prepare for hibernation. (gulls cry) With the tide falling back, the salmon are especially vulnerable in shallow water. But the sweet fresh water seems to give them new energy... ..luring them on towards danger. With the odds stacked against them, the remaining salmon withdraw, getting a free ride on the outgoing tide. A few young grizzlies follow the floundering fish back out to sea, with Mum tagging along behind. They've yet to learn how hard it is to catch a sockeye in deep water. By low tide, their prey has withdrawn out of sight. But the bears know they will be back. As the salmon begin to mass at the foot of the staircase, this adult male, in prime position, keeps stock still, so not to panic the shoal. Some youngsters move away, leaving the stage to the more experienced, older bears. The gulls gather again in anticipation. The salmon use the deeper water to retake lost ground and get closer to the stairs. More bears grab a fish and leave. But it makes little difference to the remaining salmon. To reach the river, they have to get up the stairs and on every step an adult female is ready. These older bears didn't bother chasing the salmon out to sea. They've fought for these positions. They've learned that this is the best place to stand and wait. Once again the salmon rush as one. Their only chance, safety in numbers. Cubs which can't keep still are kept back away from the fish's eyeline. Once the salmon are running, the females let the cubs join the free-for-all. With so many fish, it's all too easy for the bears. The grizzlies bite the head first, to immobilize the prey. Then with incredible dexterity strip off the fatty skin from the muscle. They abandon the flesh. And the gulls finish off. It may seem a tragic waste, but the oily skin is the best bit for building up the bears' fat reserves. And with so many fish, they can afford to be choosy. For the bears it is gluttony. For the salmon, carnage. The end of the road for so many intrepid travelers. Yet because there are so many salmon, they keep on coming and some make it past the first hurdle. They push on hard... ..stretching every sinew. To conquer the current... To leap the steps... To beat the bears... They have made it into the river. And now these feisty fish must swim on into the unknown. Their journey is far from over. Unlike other salmon, sockeyes don't lay their eggs in flowing streams or rivers. They keep on going for miles until they find a calm backwater, a pool or lake. The native Americans named this river Aniakchak, "the way out". And its wide river bed suggests that it has seen more turbulent times. Its meandering path leads to many tributaries and dead ends. Along one of these, the survivors of the local population of salmon return to their birthplace to spawn. The newcomers continue up the main river. Somehow they know not to follow the locals. They swim on in uncharted waters. Ahead of them lies Aniakchak, the volcano after which the river is named. Aniakchak is actually a seven-mile-wide basin or caldera, created by a giant volcano which collapsed in on itself. At the time of this story, the caldera is filled with a vast frozen lake, over 300 feet deep. Hidden under the layer of ice are thousands of tons of water, and several more volcanic cones, which erupt in turn. Aniakchak is the most active volcano in the Alaskan peninsula. The force of the blast shatters the ice. The wind blows the poisonous fumes away from the fish in the river, so they don't smell the danger. Under continual bombardment and shaking, the caldera's ramparts begin to crack. Water finds the weak spot. The terrifying force breaks open barriers and forges new channels down through the valley. It grabs every living thing in its path. Within hours most of the lake has drained away, but downstream the flood continues its destruction. Salmon are built to withstand the battering of currents, but not the might of a volcanic torrent. Aniakchak, "the way out", has once again recreated the landscape. Several days later, snow hides many of the mountain's scars. And the bulk of the floodwater has disappeared into the sea. But it has left behind a whole new floodplain of meanders and backwaters and dead-end pools. And into some of these it flung the salmon. Because there is so much decaying life, the water is robbed of oxygen and the few survivors struggle to breathe. Yet the urge to find fresh water is so powerful that slowly, the tenacious salmon fight on. Battered and bruised, they gradually begin to regroup. They swim on past skulls of their old enemy, the bear. They pass moose. The cataclysm has even unearthed the remains of a giant from an earlier age. A woolly mammoth tusk. The twists and turns, the maze full of dead ends scoured by the flood must seem like a point of no return. But because these individual salmon are all following the same faint mineral trail, the scent of fresh water, amazingly still potent amid the chaos, gradually they find their way back to the main channel. Now another ticking clock begins to work against them: their biological clock. They have lost a lot of time. During prolonged contact with fresh water, their heads begin to turn green, their bodies red. This red pigment, brighter in the males, helps to protect the fish from harmful ultraviolet rays. But it also signals the beginning of the end for the adult salmon. They must find a safe backwater to spawn in soon or the whole epic journey will have been in vain. The sockeyes reorient themselves to follow the chemical gradient. The call of fresh water seems to be coming from the center of the ravaged volcano itself. The salmon have actually swum through a break in the wall created by the flood, across the old lake bed into the very heart of the volcano. This small green lake, aptly named Surprise Lake, is all that is left of the frozen giant. The volcano has calmed down, but the emerald water suggests that the lake is poisonous. Is journey's end for the salmon yet another dead end? Surprise Lake collects rainwater from the caldera walls. The water gathers up a cocktail of volcanic poisons and enters the lake, hot, acidified, and toxic with minerals. The only wildlife that can survive this living hell are specialized bacteria, which were probably among the first species to colonize earth. Any other creature dices with death. And yet this is where the sockeyes have come to swim and lay their eggs. Surprise Lake hides a secret. Despite the poisonous cocktail, in some parts there's enough oxygen for salmon to breathe, even for plankton to grow. The salmon need to avoid the poisonous murk and find the source of fresh, oxygenated water. The miracle begins with the clouds, which meet the warm rock of the caldera walls and seem to melt and pour all over them. The clouds condense into droplets and seep through the rock. In some parts the water picks up poisons, but in others the ancient rock filters the water from the snow-white clouds, making it even purer and sweeter. If only the salmon can find it... This is the source. Fresh, clean, fully oxygenated water flows through a crack in the walls into the toxic hell of Surprise Lake. It is enough for the salmon. At last, several months after they were turned back from their natal river, they have found a tiny bit of paradise in the bowels of a volcano. A place where they can give their young a chance. Their appearance continues to change. The males develop hooked jaws... ready to do battle. Space is at a premium. Stray far from the source and the young salmon fry will die. The best spots are worth fighting for. The smaller, duller female chooses the winner. In the shadow of the volcano, the male guards his female, while she digs a shallow nest. Then she sheds up to 5,000 eggs, which she has carried all this way. The male covers them in clouds of sperm. And the female quickly buries them. Their quest complete, their bodies spent, the adult salmon start to deteriorate. They've kept going for thousands of miles, but the minute they've spawned, they begin to die. Alerted by the strange splashing, a red fox moves in to investigate. The fox is tempted. But the salmon aren't quite ready to give up yet and back off. A hapless ground squirrel diverts the predator. (squirrel squeaks) (squirrel squeals) Volcanic ash is a rich fertilizer. Grass grows quickly and nature moves in to fill the void created by the flood. Instinct gives the salmon enough strength for a final push to leave the lake where they spawned and head back towards the river. It's best not to draw attention to their eggs. For as their bodies continue to fall apart, the predators and scavengers will find them. The adult salmon have taken nothing from the volcano or the river. But by dying here, they leave their bodies and all the minerals within to help feed the wildlife and plant life, and ultimately to the benefit of their own salmon fry. No one knows exactly how many hundreds of years ago those first pioneering salmon found their way through the rift in the wall and into the heart of the volcano. But we know they did, because each year new salmon fry hatch out in Surprise Lake. Over the years the descendants of those original pioneering salmon adapted and acclimatized to the unique conditions in this remote, volcanic lake and became a new race of sockeye salmon. The Surprise Salmon. The young fry feed and grow for two years while the unique smell of Surprise Lake is imprinted on their brain. Then they head downstream, retracing the route of their ancestors back to the sea. Not all of them will make it. On their long, winding journey, they'll meet many new dangers. Until, at five years old, they are ready to face the ultimate challenge. Each year the adults attempt the same incredible journey back to the heart of the volcano, allowing us to unlock the secrets of those first pioneering salmon which found Surprise Lake, gave it life, and made it their final resting place. Subtitles by Silverway Media