It's a mystery that has got British law enforcement officials and others across the planet scratching their heads. Put bluntly, enough heroin to supply the world's demand for years has simply disappeared.
The United Nations Office on Drugs and Crime (UNODC) describes the situation as "a time bomb for public health and global security".
This week's Map of the Week comes courtesy of the UNODC. It shows their latest estimate of opium production in Afghanistan - another bumper year.
A crop of 7,700 tonnes will produce around 1,100 tonnes of heroin - it basically works on a 7:1 ratio.The mystery is that the global demand for heroin is less than half that. In other words, Afghanistan only needs to produce 3,500 tonnes to satisfy every known heroin user on the planet.
Look at the graph, though.
For the past three years, production has been running at almost twice the level of global demand.The numbers just don't add up.
There are two credible theories.
Theory 1: A large and undocumented market has opened up in countries which don't want to admit the problem. Russia has long been in denial over the scale of its heroin problem and the same may be true in emerging drug markets like Iran, Turkmenistan and Kazakhstan.
The Iranians are certainly increasingly anxious about the opium fields on their doorstep. Border guards and police have been involved in deadly shoot-outs with smugglers with experts suggesting that there are now a million heroin users in Iran.
But the over-supply is so great that it is hard to conceive of it all disappearing in to the blood-streams of new addicts in Tehran and Ashgabat.
Theory 2: Vast quantities of heroin and morphine are being stockpiled. Antonio Maria Costa, head of the UNODC is convinced that is the only explanation. In a recent bulletin he issues an urgent order: 'Find the missing opium.' "As a priority, intelligence services need to examine who holds this surplus, where it may go, and for what purpose" he says. "We know little about these stockpiles of drugs, besides that they are not in the hands of farmers."
Further credibility is given to the stockpiling theory in that 'farm-gate' prices for opium remain pretty stable at about $70 per kilo.
So where are the thousands of tonnes of drugs that the UNODC describe as a "time bomb"?
Well a clue, perhaps, comes from a senior law enforcement official who told me that British undercover teams in Afghanistan are reporting seizures of "enormous quantities of precursors".
Precursors are the chemicals required to turn base opium into heroin.The intelligence suggests that, rather than export opium to established drug laboratories in, for example, eastern Turkey, smugglers are processing the crop in Afghanistan.
The likelihood is that vast quantities of heroin are being warehoused somewhere close to the fields where the opium grows.
But there is another mystery surrounding the heroin market at the moment. If the international drug cartels are so awash with product that they are prepared to risk hiding billions of dollars worth, why are there shortages on some British streets?
That is the peculiar state of affairs revealed in Drugscope's recent trends survey.
"Some areas are experiencing outright shortages or shortages of good quality heroin. The quality of street heroin had dropped in 12 of the 20 town and cities surveyed, with five areas - Penzance, Cardiff, north London, Luton and Birmingham - noticing a shortage of the drug on the streets" the report says.
The field-work, conducted in July and August, finds shortages had typically been in place for two months - a longer stretch than is usual in a market well known for its peaks and troughs.
The Serious Organised Crime Agency (SOCA) believes the heroin shortage in some parts of the country could have been sparked by a rise in the price of UK wholesale heroin. "Current intelligence suggests that some criminal groups are having difficulty getting hold of what they perceive to be good quality heroin."
One theory is that smugglers are using new routes, increasingly distributing heroin through East Africa.The switch in tactics may have led to a temporary pause in supply which is being felt in the UK.
But very few would claim the shortages are the result of police activity. The Drugscope survey concludes that "street level drug enforcement had little long-term impact on illegal drug markets." At best, operations only disrupt the flow of drugs for a few days or weeks and merely displace drug use and drug dealing for a short time.
One serious anxiety is that the economic downturn will herald a new wave of drug misuse.The recession in the 80s coincided with the British heroin epidemic. In the US it was crack cocaine.
It is not just that people turn to drugs to blot out the misery of a downturn. If the crisis pushes up unemployment, it is likely that, deprived of a legitimate way to make a living, some may turn to an illegitimate source.
Perhaps a global downturn is what the drug cartels, with their huge stockpiles of heroin, have been waiting for.
Since posting this article the Serious Organised Crime Agency has been in touch.
SOCA has a number of undercover operatives in southern Afghanistan. They tell me this: "Whilst the cultivation and production of opium in Afghanistan is in decline, intelligence suggests there is considerable stockpiling of narcotics by Afghan criminal networks in order to control prices in the growing markets in Russia, China and within the local region."
I also understand that Nato's top operations commander is calling for more aggressive tactics against the opium trade in Afghanistan. US General John Craddock will tell Defence Ministers gathering in Budapest that troops should focus on "high-end" targets like drug dealers and laboratories.
Some Nato ministers, however, are concerned that any crackdown would prompt a violent backlash against allied troops.