These are all examples of significant hazards. Although hurricanes and tornadoes do occur outside the normal seasons for these events, there is clearly a period during which they must be considered significant hazards with sufficient potential for occurrence to merit increased preparedness. Earthquakes, in contrast, are not seasonal, and our knowledge does not yet allow us to identify when they transition from a hazard into a threat. We can, however, define areas in which they are a primary hazard. Dams offer an interesting hazard picture – high hazard because they impound large volumes of water immediately upstream of areas of settlement, a hazard that can be compounded into a threat by poor design, construction, or maintenance. In some cases hazards may be general and non-specific; an aircraft belonging to an airline with a notably bad safety record is not necessarily going to crash, but the past does act as a predictor of increased hazard. And some systems offer multiple hazards – power production is an example. Nuclear power plants, although generally safe, offer the hazard of catastrophic failure, and we have seen that even if the power plant is not a hazard, cascading failures can occur within the distribution system.