THE neighbours smelt it first. The heavy sickly sweet stench seeped out of the blacked-out windows of the house, a detached home in one of Glasgow's better-off suburbs.

"The guy next door phoned up and said he recognised the smell," said a detective zipping up his white paper forensic suit next to the red brick house's neat lawn. "Cannabis."

Police had raided the address the night before, one of dozens of underground dope factories uncovered in the last year. Inside were hundreds of plants.

Inside too had been a young man, the factory's sole gardener. And he had been struggling to get air fit to breathe.

"You would get high from the spores," said the detective, standing over the gardener's bed, a dirty mat in a tiny box room.

He was giving The Herald a very rare tour of a live crime scene. The little bedroom explained why. The conditions for the men, women and sometimes, it is believed, children who farm Scotland's hidden cannabis cultivations are, according to the country's top law enforcers, some of the worst they have ever encountered.

Most of the gardeners are from Vietnam, some from China. Almost all, police believe, have been trafficked into the country.

Around the bed were the man's belongings: an empty beer can used as an ash tray, some cigarettes, a change of trousers, some flipflops, a portable TV and some Chinese action films.

There, too, was a bag of orange peel and a can of air-freshener. Anything to get rid of the smell.

"What we have here is a modern day form of slavery," said Chief Superintendent Stevie Whitelock, the director of intelligence at Strathclyde Police. "We abolished slavery a few hundred years ago. But it's still here, right here in this country. We have Vietnamese being brought into the country under one guise or another.

"They are then locked up in temperatures of over 110 degrees with toxic gases and forced to work in conditions with no regard to their health and safety. These people have no freedom. It's totally unacceptable."

The Herald cannot reveal the location of the factory. But there are plenty of others, 61 in Strathclyde at last count, more than 70 across the country. Many are in detached homes, often on newish estates where neighbours know each other least.

They have all been discovered thanks to Operation League, a massive campaign by the police to drive industrial cannabis production out of the country.

Yesterday Strathclyde Police recorded their biggest result yet, a sentence of five years and three months for the man Mr Whitelock regards as a "leader", an organiser in the "command and control structures of south-east Asian organised crime groups".

Jian He, a 29-year-old student and asylum seeker, has lived in Scotland for a decade, first turning up in Falkirk in 1997. He and his associates are understood to have strong connections with the Triads, the Chinese gangs.

But they are also believed to have links with organised crime in Vietnam, home of many of the trafficked gardeners, and Scotland's own home-grown mobsters.

And they don't just grow cannabis. They are also associated with trafficking in people and drugs, video piracy and bootleg porn, and the sex trade.

Their factories, each costing tens of thousands of pounds and using sophisticated equipment, have swept across Australia, Canada and continental Europe before reaching Scotland. The business hasn't had a good reception here.

Fifty-one people have been arrested in Operation League, many on immigration charges rather than drugs ones.

Three Vietnamese gardeners were sentenced to two years each last week. Another man, 36-year-old Chinese Min Zhou, was jailed for three years in June. He had been found in a new semi in Darnley, his bed another ragged mattress and his only comfort a portable TV.

Detectives have seen worse, men sleeping in baths among the toxic fumes from fertilisers and pesticides of the plants they are tending.

In England it is believed scores of children have been smuggled into the country to look after dope, the newer and more potent varieties of skunk scientists increasingly believed are linked to serious mental illness.

Mr Whitelock has been horrified by the conditions he has seen, often in otherwise perfectly respectable-looking homes.

The one visited by The Herald still had its newbuild artwork among the four-foot high cannabis plants: Ikea prints of the Lake District.

He said: "When you walk into one of these places from the outside, it looks non-descript. It could be an empty factory, a tenement or a detached house. Inside, though, it's surreal.

"I was in one factory, an industrial premises in Shettleston. Inside there were something like 10 gazebos, the kind of thing you see in gardens, but they were on this factory floor.

"The walls of the gazebos were lined with silver foil. The floors were covered with plastic mats and there were thousands of plants at a variety of stages of growth. Four trout had been nailed through the head and stuck to the wall and this was all the farmer had to eat for himself. He had been locked into the factory with nothing but a Bunsen burner and a wok and surrounded by compost and fertiliser. It's incredible."

Scotland's underground dope factories, too, pose a major public safety risk. Three have already gone up in flames.

It was a blaze in Renfrew last year that first tipped police off that the industrial cannabis production had arrived in Scotland.

Senior fire officers expect more blazes. Dave Goodhew, Strathclyde Fire and Rescue's assistant chief officer, has issued formal warnings that otherwise ordinary looking homes could be factories, complete with electrified doors and other booby traps, like spikes under windows. Luckily no-one has died in Scotland. Yet.

The plants grow under lamps churning out more than 40 degrees celsius in packed rooms full of dust and cannabis spores.

Detectives now question why so many ordinary Scots still want to smoke cannabis, knowing that it isn't just harming them.

The drug, reclassified under Tony Blair, is also increasingly part of a major international crime ring revolving around everything from slavery to the commercial sex abuse of women.

Graeme Pearson, director of the Scottish Crime and Drug Enforcement Agency, said the jailing of Jian He was a "severe blow" to organised crime.

Mr Whitelock agreed: "It sends a strong message. My strategic aim was to make sure Scotland was a hostile place for these individuals to cultivate cannabis."