Documentary exploring the likelihood and potential global effects of the eruption of Katla, a huge volcano lurking under the Icelandic ice. Katla is five times the size of its neighbour Eyjafjallajokull, which recently caused so much travel disruption when it blew clouds of volcanic ash into the atmosphere. Historically, every Eyjafjallajokull eruption has been followed by a flare-up at Katla. If such an eruption were to happen now, it is predicted that European airspace would be closed down for 18 months.