When early in November of 1519 Cortés and his Spaniards struggled through a snowy pass in the pineclad mountains, past the elegant cones of the twin volcanoes Popocatepetl, ‘Smoking Mountain’, and Iztaccihuatl, ‘White Woman’, and made their descent into the wide shallow bowl of the Valley of Mexico, they entered a landscape unlike any they had encountered in the New World. Wide shallow lakes covered much of the valley floor. The marshland zones, speckled with the camp-settlements of fishermen and birdhunters and the low earth mounds which marked the activities of the salt-farmers, possibly looked much as they had looked for centuries. But there had been a great movement of peoples into the valley from the less favoured lands to the north from some time in the twelfth century, and that migration had transformed the land. By the early sixteenth century much of the lake edge was thickly fringed by a lacework of settlement and intensively cultivated small fields, giving way at intervals to the intricacies of substantial towns.