The bomb that Nigerian aspiring terrorist Umar Farouk Abdulmutallab attempted to ignite on Christmas Day aboard Northwestern Airlines Flight 253 was a very powerful type — one that, though small, would have likely blown a hole out of the side of the plane and potentially caused a devastating crash as it landed in Detroit. Luckily, the explosive that was sewn into his underwear — and only caused a minor fire which passengers and crew were able to put out — is extremely hard to ignite. Pentaerythritol tetranitrate, or PETN, is among the "most powerful of explosives," according to the Times, and was the same explosive that was smuggled aboard by the infamous (and also thwarted) 2001 "shoe bomber," Richard Reid.
Meanwhile, British police fear that there may be more of their former residents training in terror camps in Yemen, where Abdulmutallab prepared for his attack attempt. According to the London Sun, there are at least five radicalized British Muslim cells training in the Arab nation, each of which include at least 25 citizens. They're to return to the United Kingdom in early 2010 "and will then await internet instructions from al-Qaeda on when to strike," reports the tabloid. They're being monitored by police and will likely not be allowed back into the country. Abdulmutullab, who allegedly trained with these very cells, was denied a visa to return to the United Kingdom last May, as he was on a terrorist watch list there. "If you are on our watch list," British Home Secretary Alan Johnson pointedly observed, "then you do not come into this country."