Reaching Beichuan is a long march into hell. When you finally emerge scrabbling through the dirt into the town, what lies before you is a breathtaking vision of horror. Official estimates say China's worst natural disaster in 30 years has claimed 50,000 lives so far, but looking at the devastation here, it is hard not to imagine the final toll will be much, much higher.

Beichuan county in Sichuan province used to be home to 160,000 people, and most of them lived in the now-forsaken town of the same name, nestling in one of the world's most beautiful valleys. But everyone is gone, either dead or having abandoned their flattened home.

Beichuan was too close to the epicentre of this week's earthquake to stand a chance. At least 80 per cent of it is destroyed, with many thousands of bodies still buried in the rubble. It's hard to imagine this place ever functioning as a town again.

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There is still no access by road. People's Liberation Army soldiers rally behind red flags at a rescue station three kilometres away, before starting the trek into the heart of this shattered place.

Our journey to reach Beichuan, which has been almost completely cut off since last Monday's quake, began at a good pace but we were soon forced to slow to a solemn, single-file column as we negotiated the side of the mountain. We stopped to allow soldiers carrying bodies, and occasional survivors to pass, their faces straining with the exertion of carrying their heavy burdens up these steep slopes and across the wreckage of roads and fields.

The narrow access track up to the town is also full of villagers bringing whatever little items they can as they scramble back to the refugee camps in neighbouring towns spared the worst of the quake. These are often old people, who move slowly but steadily, in a way that clearly irritates the fit young teenage troops itching to get past. But who is going to tell a grandmother carrying her life on her back to speed up?

The first sighting of what used to be the town happens about two kilometres away. It now looks like a model railway village that a nasty child has melted and covered with sand. The town was built on the sides of the valley and when the earthquake struck, the buildings slid down on top of others in a sickening concertina, leaving most of the settlement collapsed at the base of the valley. One or two large, newer buildings survived but the other big buildings folded on houses and apartment blocks, leaving mountains of rubble dozens of feet high.

Every day I have reported the story of the Sichuan earthquake it has seemed impossible to imagine things getting worse. Hanwang, with its bodies lying everywhere, was grotesque. Dujiangyan, where hundreds of teenagers were dragged out dead from the mud, was nightmarish. But every day is worse than the next. No one knows what horrors await after Wenchuan, directly above the epicentre, is opened up. At this stage, there can be precious few survivors there. But Beichuan is a truly horrendous sight. The prospect of the death toll reaching beyond 50,000 looks increasingly likely.

Driving up along the valley, along the Chang Jiang, or Long River, which we call the Yangtze, a JCB carrying a mound of corpses wrapped in tarpaulin was an early sign that the scene in Beichuan was going to be harrowing. But it was worse, much worse, than I'd expected.

Shutters are pulled three-quarters of the way down on some shop fronts, and feet are visible beneath them, but the grocer's and fruit shops they are meant to protect are just giant mounds of debris.

At the Middle School, hundreds were buried alive, just as they had been in other schools around Sichuan. The town's prison collapsed, and who knows how many inmates died.

Negotiating the rubble is torturous; finding survivors even worse. Outside a kindergarten, parents went around calling the names of their children. A pile of small bodies had already been recovered and lay on the ground. This earthquake happened during school time, at 2.30pm.

One man found his son's body in the pile, wrapped it carefully in a plastic sheet and carried him away. His wife is a migrant worker who lives elsewhere in China and he tried to call her on his mobile to tell her their child was dead, but there was no signal yet.

"There are people alive in there, and over there, and over there," said one rescuer, outside the ruins of a hairdressing salon. "But we can't get them out; what are we supposed to do."

It's impossible not to get swept up in the relief effort; anyone who is in this town has to help, journalist or not. A crying woman said she could hear cries from beneath the rubble, and I was sent to find a stretcher and workers, but by the time we got back with assistance, the rubble had shifted and there was only a body. People hear things in earthquake zones, mistaking the wailing of the bereaved for the cries of help from their loved ones. Rescuers clamber around the debris shouting "hello" and "anyone there" but only an eerie silence answers.

The rescue effort is centred on one very small section on the edge of town, and only a tiny part of Beichuan has been explored so far. Premier Wen Jiabao, who has flown around from disaster area to disaster area in a helicopter, has visited the town twice so far, but he has been able only to voice words of encouragement to the rescue workers.

China has, cautiously, welcomed some foreign input. When Mr Wen first visited the town he spotted an American doctor, Brian Robinson of the Heart to Heart aid outfit, walking with other volunteers along the road and he ordered the car to stop. He embraced the doctor, thanked him and told him to go to Beichuan and help. An unprecedented action.

It is still impossible to get through to Beichuan with meaningful assistance and aid: most of the cranes, medical supplies and military personnel are all still back down the valley at the disaster relief headquarters.

Rescuers struggle across a bridge lying on the river bed in large chunks, a smashed car on what was the roadside the only evidence that this was ever a bridge. In the streets, the smell of corpses is accentuated by the searing heat that has replaced the heavy rain of days ago. The dry weather makes rescue work easier, but you cannot help worrying about disease being the next problem here.

At one point yesterday there was a small aftershock, causing brief panic. People are also angry that it has taken so long for the relief effort to make it to Beichuan, saying there were hundreds of people crying and shouting in the rubble even on Wednesday, but that it was too late now. There was little evidence of sniffer dogs, hi-tech equipment, and not too many helicopters around either; you would expect to see choppers ferrying supplies and people.

What they have is primitive relief work: manpower and womanpower. Thousands of soldiers and volunteers have arrived in the area, some from as far away as the provincial capital and other parts of China.

Anger, too, is growing about the poor quality of buildings. In Mianzhu, an apartment block collapsed on itself. The flats had been built using contributions from a local work unit, a group of workers organised by the Communist Party at a factory or office. Residents searching for survivors said it was because corrupt officials had demanded so much in kickbacks that the building fell. The neighbouring buildings had not collapsed, including one which housed cadres from the Communist Party. "Show me the structural steel in that building," said one woman, whose mother is missing in the rubble. "It all went into some official's pocket," she spat.

The foreign media have a poor image in China because most people now believe the international community does not want China to host the Olympics and is planning a boycott. This makes reporting the disaster difficult; before the anti-Chinese riots in Tibet, and the sympathetic view of Tibetans in the Western media, foreign journalists were popular. Now we are seen as a threat. But the soldiers are still helpful, even offering to lift us over the worst of the holes.

All privately owned cars are stopped at a roadblock 20 kilometres from the town to give better access to emergency vehicles. We leave our car and start to walk, but we are picked up by a trio of relief workers in a blue CDW truck, laden with baskets for carrying debris. For the first time in four days of reporting the Sichuan earthquake, I see the classic quake image, the fissure stretching the length of a road, made by tectonic plates shifting and tearing the Tarmac apart.

Later even this truck is stopped and we walk the remaining three kilometres with volunteers from nearby Mianyang, bringing money, clothes, food and water. "We are all one family and one nation and when our friend suffers, it's our duty to help," says Lu Fushan, 49, a farmer. His truck was stopped, as were many of the volunteers, because there is not enough room for the material to get through.

Dotted along the narrow mountainside road are smashed cars and rocks the size of lorries, which limit traffic to one lane. There are hundreds of military vehicles along the roadside.

Back along the valley, a couple sit clinging to each other on a bench, looking at the wreckage of their house. "Our house is smashed, broken," says Fu Youjun, 34, a farmer. "My father is in the hospital. We felt the quake, we were terribly scared and we rushed out. When we came out we just saw white smoke and dust.

"My brother is in town, but I can't reach him. Lots of friends and relatives. We can't get in touch with them, we tried to get through to them but there's no road. And the water is rising, there could be flooding too. I don't know what we'll do next,"

His wife, Wang Hongmei, 30, says: "The house is slipped, subsided. My nephew is in there, over there in Beichuan. What are we going go do? We just got married this year."

But even amid the heartbreak of Beichuan there is an occasional story of hope. "Don't help me, help the others, they need it more," one woman said, as soldiers bore her on a stretcher into a makeshift first-aid centre, seconds after pulling her from the wreckage. She had lived through 72 almost unbelievable hours under the rubble.

At the first-aid station, there is no time for medics to find the woman's name. She looks a youthful 40, wearing a pale blue top and smart trousers. She is weak and delirious, but so far she is alive. The woman is carried on a stretcher back across the mountains by eight strong, sure-footed young PLA soldiers.

Back at the rescue station, rescuers form a cordon for the returning stretchers. When a survivor is brought through, the soldiers cheer, and when yet another stretcher comes through with a corpse there are disappointed murmurs and sad comments.

The woman is brought through on the stretcher, a scarf covering her face, and the response is at first muted, depressed. Then, she raises her hand and waves; the scarf is there to protect her from the hot sun. Some of the PLA's most hardened troops laugh and cheer like schoolboys. This one is alive.

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