I've posted this before but it's still very relevant-
"The explosive that nearly brought down Northwest Airlines flight 253 is extremely powerful, allowing terrorists to use only small quantities to cause enormous damage. And the colourless crystals of the substance, PETN or pentaerythritol trinitrate, are hard to detect if carried in a sealed container.
Richard Reid, the "shoe bomber", tried to set off a PETN device on an American Airlines jet to Miami in 2001 and this summer a man tried to assassinate a member of the Saudi royal family after evading security detectors by hiding a PETN-based bomb inside his body.
"If you can lay your hands on a reliable source, it would be the explosive of choice," said Hans Michels, an explosives expert at Imperial College, London. A little more than 100g of PETN could destroy a car.
The device allegedly used by Umar Farouk Abdulmutallab involved a syringe and a soft plastic container filled, reportedly, with 80g of PETN. The remnants of the bomb are being analysed in an FBI laboratory.
PETN is relatively stable and is detonated either by heat or a shockwave. It is possible that the suspect used a syringe that was converted into an electrical detonator, but more likely the syringe was filled with nitroglycerin.
Abdulmutallab was in a window seat, 19A, and allegedly had the device strapped to his left leg, against the body of the plane. The idea was almost certainly to blow a hole at much higher altitude, so that the decompression would tear the aircraft apart.
"There was almost certainly a failure between the primary and the main charge that meant the PETN did not fully detonate. If it had, the plane might have limped home because it was already quite low," Michels said.
If the syringe used a liquid detonator, such as nitroglycerin, the device would have been extremely hard to detect through the usual airport security measures. Abdulmutallab cleared security in Lagos and Amsterdam after passing through a metal detector and having had x-rays of his hand-luggage.
Michels said that even more advanced security measures, such as machines that "sniff" passengers for explosives or analyse swabs of their clothes and belongings, might not have spotted the device."