Also known as "magic marble", it is putty-like and can be cut to size and moulded.

In the 70s and 80s almost 7,000 tonnes of Semtex was exported to Libya, Iraq, Iran and North Korea. The downing of Pan Am Flight 103 over Lockerbie in 1988 was thought to have been caused by Semtex.

The IRA is believed to have bought 2.75 tonnes from Libya's Colonel Gadhafi. Real IRA suspects were arrested last year in Slovakia allegedly trying to buy Semtex. Semtex was originally odourless, but a smelly version has recently been invented that can be detected by sniffer dogs. Metal shavings have also been introduced to make it more detectable.

The explosive has a lifespan of up to 20 years, meaning terrorists can stockpile it.