A t one time, scientists thought that mountains and valleys arise because the earth is shrinking; all the wrinkles, folds, faults and other deformations in the planet's crust were likened to the crinkly skin of a drying apple. Then came the theory of plate tectonics, which revolutionized the earth sciences and gave researchers a more tenable framework for understanding a remarkable variety of geologic processes. According to this theory, the continents and oceans are embedded in a dozen or so thin plates that make up the earth's mechanically strong "lithosphere" and that float like rafts over the weak, partly molten "athenosphere." Mountains are created when two continental plates collide; the Himalayas, for example, have been built over the last 50 million years by the Indian plate's relentless drive into Asia. But plate tectonics, too, has its faults. While the theory is very successful at explaining how oceans and oceanic crust form, in many ways it leaves the continents high and dry In the case of the India-Asia collision, for instance, it doesn't explain why the oldest part of the Indian plate has survived the collision intact while sections of Asia have been violently deformed. Plate tectonics also assumes that plates are no thicker than about 100 kilometers and that the structure of the lithosphere and athenosphere beneath the continents is essentially the same as that underlying oceanic crust. Over the last several years, however, many scientists have come to think that the plates under cratons the oldest continental cores, which have remained undeformed for more than a billion years are much thicker than 100 km and may indeed have temperature and chemical profiles that differ from plates under oceans. While researchers are still debating exactly how deep these "continental roots" extend, the evidence for thick continental plates is challenging traditional models of how the continents evolved. And recent studies suggest that to understand continents' surface behavior, such as the India-Asia collision, one must look at the deep structure of cratons, which billions of years ago somehow shepherded and stabilized the normally mobile mantle material beneath them.