Since the Great Depression, the United States has not experienced anything like a real meltdown, though at times markets have seemed to be spinning toward one. But the fear of a financial Armageddon has inspired an imaginative literature that at the extreme has its parallel in natural-disaster plots, mass-computer-failure sagas, and the like. In the abstract, no one can say whether systemic risk presents a real threat—the very phrase is vague and ill defined—but regulators naturally err toward caution.
