In 1519 Cortés began his invasion of the Aztec empire. By 1520 Montezuma would be dead, and by 1521 the empire would have fallen. Within the next half dozen decades 95% of the Aztecs would be dead of disease. But Montezuma and the Aztecs had almost no warning of the cataclysm that was about to befall them. Is there some cataclysm waiting in our future which we will similarly be completely ignorant of until it is upon us? Probably. If that's the case what measures could we possibly take?