Plastic explosive, manufactured in the Czech Republic. It is safe to handle (it can only be ignited by a detonator) and difficult to trace, since it has no smell. It has been used by extremist groups in the Middle East and by the IRA in Northern Ireland.
0.5kg of Semtex is thought to have been the cause of an explosion that destroyed a Pan-American Boeing 747 in flight over Lockerbie, Scotland, in Dec 1988, killing 270 people.