Step over a wall, slide down an embankment and tread carefully along a slime-covered path and you are in Habib Nalah - a shadow world of heroin addiction fuelled by the opium boom across the nearby Afghan border.

More than 500 men, from teenagers to grandfathers, are gathered at the edge of a giant drain that snakes through central Quetta. The scene has a nightmarish quality. Black sewage water oozes down the middle and dried excrement litters the path that runs alongside. A suffocating stench clogs the air.

Clusters of addicts hunker by the stream, under a line of gloomy pillars supporting the shops overhead. They are "chasing the dragon": delicately burning brown heroin powder on foil paper, sucking the pungent fumes with a pipe, then slouching backwards, eyes rolling.

In one corner a moaning man is crouched in a ball, head gripped in his hands. Whether immersed in a personal heaven or hell, it is hard to tell.

Another begs money for treatment. "Just one last chance," pleads Muhammad Daud, 22. "Otherwise, just shoot me now."

Record crop



Most of this year's bumper opium crop from Afghanistan - worth a record $2.8bn (£1.45bn) - will find its way to lucrative European markets. Peddled on back streets from Manchester to Moscow, it will cause more than 10,000 deaths, according to a recent report by the United Nations Office on Drugs and Crime.

But as the drugs pass along age-old smuggling routes, some spill over into local markets, with devastating consequences. Pakistan has about 500,000 chronic heroin addicts; Iran has 1 million, according to the UN. The problem was born of the 1980s opium boom. Now it appears unstoppable.

Junkies from across Pakistan flock to Quetta, 60 miles south of the lawless border. Here heroin costs just 70 rupees (60p) for a sachet, and the supply is abundant. Muhammad Jawaid arrived from Multan, 500 miles to the south, seven months ago. Once a painter, he now collects rubbish to feed his habit.

"At home it costs 200 rupees for a sachet," he says. "Still, I want to go back. But I can't."

Habib Nalah's heroin dens have an air of permanency. Sheets strung between the pillars divide the drain into compartments. Ragged posters of Indian film stars are pinned to the greasy walls. Vendors sell sweet tea, matches and tranquilisers. A whiff of hashish drifts through the air.

The addicts wash under a leaking water pipe. The "saqis" hover nearby. These are the dealers who divide up the heroin smuggled from Afghan refugee camps. They also mediate disputes, bribe police and maintain social order. When an addict dies a saqi pays four men one sachet each to haul the body up to the street.

Up to 800 addicts live here during the summer season, says Aftab Ali, a courageous social worker known by all the addicts. There are no age limits. One compartment has grandfathers with whiskery beards and kindly smiles. The next has a teenage boy.

Nassir, a grubby-fingered 15-year-old, says his mother carried him here nine years ago. Now she is dead, Nassir has become an addict, and Habib Nalah is home. "My brother has given up, but not me," he shrugs. "Can you give me some change?"

The flood of Afghan opium has swamped Pakistani security forces. Breaking the supply pipeline is near impossible, says Brig Anwar ul Haq, head of the Anti-Narcotics Force for Balochistan province.

Smugglers flit across the long, leaky desert border using ancient wiles and modern technology. Donkeys, camels and heavily armed convoys carry the drugs. The criminals outfox border surveillance with satellite phones and, if necessary, shoot their way out of trouble with rifles and rockets. "They are highly mobile and technically advanced," he says.

In 2004 the Balochistan force confiscated seven tonnes of morphine and 157kg (346lb) of heroin. In contrast, Afghanistan's opium harvest was estimated at 4,200 tonnes, and the traffickers earned $2.2bn.

One problem is the lack of cooperation between security forces. "Until intelligence is shared across the border we are always in a reactive mode," says Brig Anwar.

Bribery is another weakness, admits Shoukat Haider Changezi, chief of the Levies, a tribal-based rural police force. "An officer earns 3,500 rupees. Then his cousin, who is from the same tribe, offers him 10 times as much to turn a blind eye. It's a fortune," he says.

The best heroin - known as "rose quality" - is re-exported from smuggling ports on the Arabian sea, or spirited across the border into Iran. The lower-grade drug is sold locally.

HIV infection



Soaring HIV rates linked to increasing needle use is the latest worry for authorities in Karachi, Lahore and smaller cities. In Habib Nalah most addicts still "chase the dragon", but say they want to stop. The lucky ones make it to the Milo Shaheed Trust, one of five rehab centres in Quetta. The "cold turkey" treatment is tough - three months of forced withdrawal with only multi-vitamins as a substitute - and the relapse rate is 75%.

Abdul Rehman is in rehab for the 15th time. A junkie since 1978, he checked in 70 days ago. "This is my last chance," he says.

Muhammad Zia, 26, came from Ghazni in central Afghanistan. "A friend introduced me to heroin five years ago. He said it's like entering paradise. But believe me, I have gone through hell," he says.

Mr Zia came to Quetta after his father expelled him from home. He knew dozens of other addicts, he says. "There are many people with the same problem, even women, in Ghazni. They just stay in their houses and smoke, smoke, smoke."

For some, though, there is no last chance. The following day Mr Ali, the social worker, carries the body of Mohsin Ali to a local mosque to be washed before burial. The 28-year-old Afghan refugee died earlier in the morning, four days after entering the Milo Shaheed centre.

"He had been eating and smoking opium for years. Yesterday he became very sick with swollen lungs," says Mr Aftab, carefully pulling the blue gown off the corpse. "We called the doctor to come and help. But it was too late."