Throughout the centuries human curiosity about the unknown has led individuals on adventures to the far reaches of the globe. Ancient exploration was largely in the context of military conquest. Perhaps the best early example is that of Alexander the Great (356-323 B.C.), whose exploration created an empire was so vast that it remained unmatched for more than a thousand years, until the Vikings set out across Europe and the Atlantic. The Roman Empire also expanded its borders—to the north as far as Britain (Albion) and to the south as far as the Atlas Mountains in northern Africa—but with a greater interest in colonization, not exploration. In addition to conquest and colonization, the search for new routes to commerce, especially for the luxury commodity of silk, and new opportunities for religious conversion prompted exploration. The Chinese ventured...