At one time odourless and the colour of orange Playdough, it was, along with beer and a certain make of cheaper Eastern European car, one of the most favourite exports of communist Czechoslovakia.
Semtex was and is a plastic explosive that made its way around the world, eventually ending up in large quantities in Colonel Gadaffi's Libya.
It was, of course, Gadaffi's support for the Provisional IRA in the 1980s that led him to ship tonnes of the explosive via three huge arms shipments to Ireland. The gift from the dictator massively boosted PIRA's arsenal, enabling it to set off bigger, more devastating bombs than before.
With Semtex as the "trigger" explosive, IRA engineers were able to detonate huge 1,000lb-plus devices which wreaked death and havoc in places like Bishops Gate in and Canary Wharf in London after the ceasefire broke down in 1996. Here, the explosive was used in under-car devices which killed off-duty police officers, soldiers and loyalist enemies from the mid-80s to the mid-90s. Semtex became synonymous not with the Czech factory that made it, but rather the Provisionals and their terror campaign.
When PIRA decommissioned most (certainly not all) of its arsenal a decade ago as part of moves towards power-sharing and devolution, it was assumed that this most lethal of weapons would be put beyond use. Not so, it now seems.
No one should really be surprised that half-a-kilo of Semtex was uncovered during the security operation in Ballymurphy on Thursday night. There have been several instances over the last decade when the explosive has been discovered, including 15kgs of Semtex found at a site near Dublin Airport in 2013, which Garda believed belonged to dissidents.
Clearly not all of the Semtex donated by the Gadaffi regime ended up being put beyond use to satisfy unionists that the IRA was going out of the terror business. One reason for this may be to do with the IRA's quartermaster department in the 1990s; the unit that looked after the huge, carefully constructed network of bunkers and hides where Semtex was concealed. It is worth remembering that the man in charge of that department when the Provisional IRA divided in the autumn of 1997 was Michael McKevitt, later the founder of the Real IRA. He and those allied to him at the time may have secreted a quantity of weapons, including the deadly explosive, just in case the republican movement's leadership ultimately decided to disarm in their long road to constitutional, non-violent politics. Which, of course, is exactly as it turned out to be.
Although PIRA had a tightly controlled national command structure, there had been instances in the past where individual active service units jealously guarded their own armouries and kept them from leaders in Belfast or Dublin. In areas such as east Tyrone, for instance, Semtex was later used by dissident republicans to murder a young PSNI constable, with the suspicion that the explosives were part of the arsenal "held back" by semi-independent military units.
What is highly unlikely is that someone high up in the PIRA structure, which Chief Constable George Hamilton says still exists, would gift any of the dissident republican groupings Semtex to keep their wars going. While mainstream PIRA veterans might have maintained weapons of self-defence against the dissidents, they were hardly likely to hand over weapons of war to people who now regard them as traitors. This charge, if it comes from unionist quarters, can be easily dismissed.
After communism was overthrown and the Husak dictatorship toppled as a result of one of the most glorious events in European history - the peaceful Velvet Revolution - the new democratic Czechoslovak Government of Vaclav Havel promised to ban the export of Semtex. It also ordered its manufacturers to inject a chemical into the explosive that no longer made it undetectable to sniffer dogs' noses.
Yet only two years after the revolution, Semtex was being exported once more on the global market and being sold principally for use in civilian demolition work. So, the idea that there is no Semtex out there that has been stolen or sold on and cannot be bought on the black market is absurd. Someone, somewhere is ready to flog this lethal Playdough-looking substance to the dissidents of every hue in Irish republicanism still.