When disasters happen scientists pretty much have a duty to try to understand what happened and why, and to try to learn the lessons. This week the catastophist Gordon Woo of Risk Management Solutions gave a seminar here at the Cabot Institute and suggested that the question that we should really ask is not "why did this happen?" but "why did this not happen before?". This is also one of the ideas that emerged from a recent exercise that we undertook to try to understand the recent events at the Fukushima nuclear power plant in Japan. The range of skills available within Cabot allowed us to take a fundamentally holistic approach to the analysis that wouldn't have been possible for any single individual. The results of the analysis are here, but two main points emerge. First, there is the need to tackle is "chained" or "cascaded" hazards, which, as very low probability events, have traditionally been treated as independent random events