Six armed robbers. One warehouse. Three tons of gold worth around $320 million today. The raid on the Brink's-Mat secure storage facility on the edge of London's Heathrow airport on 26 November 1983 was Britain's biggest ever gold robbery. The crime caused a sensation in the press as headlines posed the question everyone across the country was asking: what happened to the Brink's-Mat gold?

A year before the robbery, a gang of professional robbers from south-east London, led by a man calling himself 'the Colonel', had set their sights on the Brink's-Mat warehouse. Inside was kept gold, silver, gems, cash, and basically anything else of extremely high value and little volume. These assets came in and out of the warehouse on a daily basis via flights at Heathrow. The problem was the warehouse was a seriously tough nut to crack, with alarms linked to the police and other security firms, timed locks, and a huge ultramodern vault that required multiple combinations and multiple keys to open. The thieves, though, had the best key of all: a man on the inside. Security guard Tony Black had been nobbled. Black was paid handsomely to let the gang in through a small and poorly secured back door just after 6:30 a.m.

When the hooded and armed gang got inside the warehouse, they rounded up the shocked guards who were just then making their morning cup of tea. Time was crucial, and the gang was not here to mess about. Thanks to information from Black, the gang knew exactly which two guards had the combinations and keys to the vault. The pair were tied up, hooded, and petrol was poured over them. The option was simple: open the vault or burn to death.

With the vault doors open, the gang began to work on the three locked safes inside. Try as they might, the thieves could not open what was surely a treasure trove of precious metals and jewels. The guards, in their terror, were in a high state of confusion, and their dual keys and combinations were not working. One guard simply could not remember the latest combinations, which were changed regularly by the security firm. The thieves knew the safes could not be forced since magnetic plates inside would then shift and trigger the alarm systems. Cursing their lack of success and about to leave empty-handed, one of the gang investigated what seemed like a pile of shoe boxes on the floor. Removing a portion of the cardboard cover, the unmistakable glint of gold shone through. It seemed incredible, but there, lying on the floor, were 75 boxes of gold worth £26 million at the time. The gang loaded up their two vans and drove off, literally laughing all the way to the bank.

When the news broke of the sensational robbery, gold prices immediately rose by around 7%. The gang had already increased the value of their haul by nearly £2 million. But it was all too good to be true for the criminals. The enormous size of the haul meant that the government and the police would stop at nothing to get back the stolen property. A crime of this size could not be seen to pay. The famed detectives of Scotland Yard and the Flying Squad were called in. The Customs Service was also involved in the likely event that the gold was smuggled out of the country and perhaps even back in again in a different form. The insurer Lloyd's of London offered a cool £2 million for any information regarding the robbery.

The police investigation quickly focussed on the only possible explanation for a successful robbery on such a high-security installation. One or more of the guards must have helped the thieves. Interrogated, Black confessed; his brother-in-law was one of the gang. Black, whom the press called the 'Golden Mole', was sentenced to six years imprisonment. Descriptions of their voices from the other guards and members of the public describing a heavily-laden van leaving the warehouse on the morning of the robbery helped the police identify some of the criminals and catch them, including 'the Colonel'. Next was to trace the dealers whom the criminals had offloaded their gold to – the part of the investigation that hinted at the extensive tentacles of international organised crime groups.

The vast majority of the stolen gold was never recovered. Just 11 bars were found buried in a garden. The rest, melted down and recast to hide its origins, was kept in ingot form or sold to dealers as scrap gold, which was then cut up, resold, and reworked into jewellery. The dealers found themselves with carrier bags of cash, which they deposited into various banks with no questions asked (a subsequent law obliged banks to show more curiosity in the future). The seemingly legitimate jewellery businesses and fences cooked their account books. But such quantities of gold were difficult to hide, and the authorities began to follow a trail of false accounts that did not add up or revealed unbelievable spikes in legitimate business.

With such a huge amount of wealth, organised crime groups were certainly involved as the gold and cash disappeared, laundered into a maze of offshore accounts, fake companies, and dodgy property deals. The police traced possible involvement to known criminals in London, Jersey, Continental Europe, and the United States. A few of those who had handled the stolen gold, the smaller, more careless fish, served prison time. Most of the successful convictions were based on tax evasion on the bullion rather than the theft itself. The bigger fish got away.

Most of the stolen gold would have eventually found itself back on the open market in one way or another. It is thought that anyone who bought jewellery in London's Hatton Garden shops in the 1980s had a 50:50 chance of wearing gold from the robbery. After suffering a severe dent in its gold stock, Johnson Matthey Bankers, the original owners, very likely bought some of the stolen gold back again. Demand for gold, still considered the surest way to keep wealth, remained as high as ever. As Bryan Boyce, the detective in charge of the investigation into the Brink's-Mat robbery recalled, “Gold has a strange effect on many people. Gold has no conscience. And some people who deal in it have no conscience either”.