Ash from Iceland's spewing volcano halted air traffic across a wide swathe of Europe on Thursday, grounding planes on a scale not seen since the 9/11 terror attacks. Thousands of flights were canceled, tens of thousands of passengers were stranded and officials said it was not clear when it would be safe enough to fly again. In a sobering comment, one scientist in Iceland said the ejection of volcanic ash – and therefore disruptions in air travel – could continue for days or even weeks.