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This Article develops a framework to govern the interactions of nations cooperating to mitigate the threat of unexpected natural disasters that potentially could affect them all. It uses asteroid impact with the Earth as the representative example because this is an "unusual and extreme" disaster that has created difficulty for lawmakers due to its many unanswerable questions. By explaining a number of the legal requirements necessary to mitigate such threats, this Article identifies legal principles that apply equally to natural threats throughout the global commons. The law involved in this analysis arises, in great part, from the judicial recognition of a state's duty of self-preservation. To this end, the American civil defense experience provides guidance on the extent of a government's duty to plan for the unknown, as does the Vorsorgeprinzip in Germany. At the international level, the doctrine of "cooperative preservation" requires nations to cooperate in joint preventive actions to mitigate dangers so great that no single country alone could effectively protect its citizens from the harm. In order to obtain greater insight into developing an international framework for mitigating extreme natural disasters, this Article considers examples of the law applied to existing efforts to mitigate transboundary nuclear and biochemical disasters, multinational forest fires, and the spread of new and re-emerging infectious diseases across international borders.