It was one of Scotland's worst industrial disasters which killed nine workers and left scores injured.
Experts traced the cause of the explosion to a corroded pipe
The explosion at the Stockline plastic factory in Glasgow generated headlines across the world and triggered a massive rescue operation.
But the devastation which ripped through the site in the Maryhill area of the city was triggered by something as simple as a leaky pipe.
Health and safety experts have spoken for the first time about their extensive £1m investigation into the blast.
Stuart Hawksworth, of the Health and Safety Laboratory, revealed the cause of the explosion.
"There was an explosive atmosphere in the cellar of the building which was caused by a leak of gas from a liquid propane gas pipe," he said.
"The actual leak was just outside the building which had gone through the wall into the cellar area.
"It built up to the right concentration to give you an explosion. That then ignited. The explosion lifted the floor.
"That then released the explosion into the upper floors of the building, pushing out the walls, lifting the floors and the building collapsed as a result."
The investigation into the tragedy was the biggest of its kind undertaken by authorities in Scotland.
For the first time, prosecutors, police and the Health and Safety Executive
worked jointly to examine the cause of the explosion and subsequent collapse of
They eliminated natural gas, which fuelled one of the ovens, and ruled out the
age of the building and any dust or solvent explosion.
After boring holes into the ground, they discovered propane from liquid petroleum gas (LPG) which led them to the pipe from the LPG tank which powered an oven on the first floor.
The evidence pointed towards an explosion in the basement. Explosives specialists, metallurgists and mechanical engineers carried out meticulous tests which confirmed a join
on the LPG pipe had corroded.
As the investigation continued, it emerged that the seeds of the tragedy were
sown as far back as 1969.
At that time, a propane LPG tank and pipework, some of it running
underground, was installed at the factory.
But part of this pipework came up vertically out of the ground before entering
Five years later, at about 1974, the yard outside the building was raised,
burying this exposed section of pipework.
All sections of the buried pipework deteriorated and corroded, as did a
cast-iron bend joining two sections of the pipework outside the building
This led to an escape of LPG which accumulated in the basement area in a cloud
which ignited, and exploded.
Stewart Campbell, HSE director for Scotland, said: "We carried out a review of potential
sources of fuel for the explosion and established individual lines of inquiry.
"It wasn't just a question of making sure we identified the cause - it is
eliminating all the other possibilities.
"All that took a lot of time but it had to be done very carefully. At the same time, all the interviewing of potential witnesses was ongoing which we did in conjunction with the police."
Philip Heyes, head of the lab's investigation team, said: "What made this so
complex was the enormity of the event, the amount of debris, and that had to be
sifted through, at the same time trying to preserve the evidence.
"The other element in the complexity was in the range of the scientific
disciplines to investigate all the possible scenarios."
They found that just five litres of the LPG, which had gathered as a gas in the
basement, was enough to cause the blast.
Mr Campbell added: "We don't want an incident like this to pass without
lessons being learned.
"It's important that lessons are learned and are seen to be learned."
Mr Hawksworth explained that it took thousands of hours of painstaking work to piece together the cause of the disaster.
He said: "We wanted to prove that there had been an explosion and the cause of it. Finding the leak took some time.
"The pipe we were looking for was buried deep beneath the ground and there were thousands of tonnes of rubble.
"We had to move it very carefully and carry out a careful investigation."