We often speak of America as "unknown," except to its own inhabitants, in the Middle Ages. But so, in a sense, was Europe, which hardly figured on the maps and in the calculations of the immensely richer, more populous, and technologically more advanced civilizations of maritime Asia. Some Europeans tried to make their fortunes as peddlers along the teeming trade routes of the Indian Ocean, but, in general, Christendom could look to the Orient only in covetousness or despair. Increasingly, in the late fourteenth and fifteenth centuries, some communities along Europe’s western edge—mainly in Portugal and Spain, but also, to a lesser extent, in France, England, the Netherlands, and Denmark, turned to exploration of the Atlantic in a search for resources—rather like "Third World" countries today, drilling offshore. But the prevailing westerly winds seemed to pinion them inside the area east of the Azores.

Meanwhile, within the Americas, most peoples seem to have remained confined to their ecological comfort zones and never to have built up much knowledge of each other’s territories, much less a picture of the whole hemisphere or of any substantial part of it. Even in the great civilizations of Mesoamerica, mapping was local or regional, and existing contacts—with the Caribbean region, the Mississippi valley, the North American southwest, and Central and Andean America—were sporadic and tenuous. When European explorers finally crossed the Atlantic, they relied absolutely on natives to guide them around the Caribbean and along inland routes and provide them with the rudiments of maps, but the construction of an overall image of the Americas was impossible until the outsiders, with their broader perspective, arrived.

European navigation reached across the ocean with surprising suddenness in a single decade. The breakthrough of the 1490s owed nothing to new technology. Though rigging and the waterproofing of casks improved in the late middle ages, the improvements were slow and incremental. Past historians’ belief in the revelatory effects of new direction-finding technology—quadrants and astrolabes—has proved false: we now know that no navigators used them effectively, but relied on primitive celestial navigation to find their latitude out of sight of land: estimating the elevation of the Pole Star or sun, or watching the passage of the guard stars around Polaris. Nor did any sudden new "spirit of adventure" inspire them, though the typical "pulp fiction" of the late Middle Ages did provide explorers with role models: the knights errant of chivalric romance and, to a lesser extent, the pilgrims and exiles of hagiographical literature. Nor did missionary idealism or scientific curiosity influence exploration, except at the level of rhetoric. The main reason for the accelerations of exploration was financial. In the 1480s, returns on investment in the Azores, the Canary Islands, and the North Atlantic fisheries accumulated to the point where investors were willing to finance voyages farther afield. Over the entire period covered in this article, private enterprise funded exploration.

Columbus was the first explorer to benefit from the new investment climate. Italian bankers in Seville and a consortium centered at the royal court of Castile backed his project for an attempted Atlantic crossing. His contract specified an impossible objective: a short route to the Indian Ocean. Instead, he found an obstacle course of islands and mainland. But in voyages of 1492 and 1493 he did open viable routes to and fro across the ocean. His achievement was the result of extraordinary daring: sailing with the wind at his back—a practice modern yachtsmen love but medieval explorers avoided: only by sailing into the wind could they guarantee their passage home.

Few commentators endorsed Columbus’s claim to have got near—at least—to Asia. Venetian John Cabot was one of them. He reasoned that if Columbus could get across the breadth of the ocean at 28° north, where the globe was relatively wide, a much shorter crossing might be possible farther north. In 1497 Cabot explored the Newfoundland coast, insisting he had discovered "the country of the Great Khan." Portuguese and Bristolian expeditions between 1498 and 1502 explored the coastline from Hudson Strait to Nova Scotia and perhaps beyond, stimulating Portuguese and Spanish mapping of the coast as far south as Florida in defensive response.

Columbus’s shipmates, Vicente Yañez Pinzón and Alonso de Hojeda, were the most enterprising of the many explorers who extended his findings along America’s Atlantic and Caribbean coasts. In late 1499 Pinzón arrived at the mouth of the Amazon (which he mistook for the Ganges). Another henchman of Columbus, Amerigo Vespucci, left descriptions so vague, sailing directions so amateurish, and calculations so wild that it is impossible to be sure by what routes he sailed or how far south he got. On a voyage probably of 1501–1502, he reached at least as far as Rio de Janeiro, encouraging successors to continue the reconnaissance of South America.

Portuguese explorers, meanwhile, had opened a new route to the wealth of the Indian Ocean around the Cape of Good Hope. But a Portuguese malcontent, known in Anglophone tradition as Ferdinand Magellan, shared Columbus’s picture of a small world, in which Asia lay only a short way beyond America. He shifted allegiance to Spain when Portuguese interest in further westward exploration ebbed. When he finally found the longed-for strait that led to the Pacific in 1520, he had in effect already failed: the strait was too southerly to be a convenient gateway. The maze-like web of channels tortured explorers. Magellan followed the Humboldt currents for three weeks along the Chilean coast before dashing across toward the Philippines. Thereafter, exploration of South America’s Pacific coast proceeded from the north.

By then, the objectives of Spanish explorers of the mainland had changed. Encounters with Native peoples conjured rumors of rich civilizations in the interior, and—without displacing obsessions with the search for a route to Asia—inspired Vasco Núñez de Balboa to seek the realm of Dabeiba along the San Juan valley in 1512. On a second voyage in 1513, Balboa crossed the Isthmus of Panama to "gaze on the Pacific." Hernán Cortés followed on what was initially intended to be a reconnaissance expedition to Veracruz in August 1519. Abjuring the authority of his superior in Cuba, he marched inland in search of Aztec riches. The route was consciously chosen to penetrate the most inaccessible parts of the Aztec world, where the Aztecs’ most reluctant tributaries and most defiant enemies would be found. As a conqueror, Cortés is overrated: a coalition of indigenous peoples overthrew the Aztecs. As an explorer, however, he extended the existing routes of contact between civilizations in world-changing fashion. The great belt of rich sedentary civilizations that stretched across Eurasia could now begin to exchange culture and biota with those of the Americas.

Conscious imitation of Cortés, plus the usual fantastic ambitions, animated the conquistadors of Peru. The illiterate Francisco Pizarro was to become "governor, captain general and adelantado," and his officers would be noblemen and high bishops. After climbing the Andes, Pizarro followed the Inca road that crossed from Cajatamba into Jauja before descending, in late October 1533, to Cuzco. The riches he garnered attracted more explorers to the Andes. Three expeditions from different directions met at Bogotá in 1539 and started rumors of the existence of "El Dorado"—not a place but a gold-rich chieftain, whom other explorers sought for the rest of the century in the Venezuelan lowlands and highlands of Guyana. Meanwhile, the search for a fabled "land of cinnamon" led one of Pizarro’s brothers to the Amazon, the length of which Diego de Orellana sailed in 1540.

North America had equally persistent legends. In 1539, a missionary’s black servant, overtaken by disease and delirium, left a garbled note telling of great cities with emerald-studded temples. In April 1540, Francisco Vázquez de Coronado led an expedition of 200 horsemen and 1,000 slaves in search of them. After two grueling months, they reached the Pueblo civilization—"good people," the Spaniards reported, more devoted to agriculture than war. Coronado pressed north to a supposedly rich, urban culture in what is now Kansas. The vaunted cities were no more than turf lodge settlements of the "raccoon-eyed," tattoo-faced Kirikiri.

An impatient and profligate explorer, Hernando de Soto, thought he could do better than Coronado by tramping overland from Tampa Bay. Immiserated, diseased, and defeated, the expedition fled after De Soto’s apparently natural death in May 1542. The failure discouraged Spanish efforts in the North American south but by the 1590s a series of independent expeditions had reconnoitered New Mexico. Surviving accounts of Juan de Oñate’s expedition to found a colony there in 1598 describe the obstacles. Gaspar Pérez de Villagrá recounted sun so fierce that his eyes "seemed to burst from their sockets," the horses were blinded, and men "breathed fire" from the desert heat. The colonists hoped to find an overland route to link up with the California coast, which Spanish navigators had begun to map in the 1540s, but the terrain and distance proved unconquerable.

On the colony’s eastern flank, neglect of the Mississippi made it impossible to create the kind of network of river routes that was emerging in South America. In the North, however, the usefulness of the St. Lawrence as a causeway toward the interior emerged relatively early. Here, as in the cases of the South American river routes, legends of rich kingdoms became the explorers’ main inducement. In 1535 Jacques Cartier traveled down the St. Lawrence in search of the fabled riches of the Saguenay. At Hochelaga (site of present-day Montreal), some 2,000 people flocked into the geometrically perfect streets to greet the French explorer. Yet he found it difficult to sustain cordial relations with the Native Americans, and his search for the Saguenay’s riches proved fruitless.

As in Spain’s dominions, French expectations of what the New World offered gradually changed. Furs replaced gold as the commercial objective. In 1600 the Crown granted a monopoly to a consortium from Honfleur and Dieppe; felicitously, the promoters chose Samuel de Champlain as governor, who opened access to the interior via the St. Lawrence as well as the Mississippi-Missouri system. English and Dutch explorers, meanwhile, found America frustrating and disappointing. They were torn between two objectives: founding colonies in imitation of or competition with Spain, and by-passing America, or finding some quick route across it, in pursuit of Columbus’s old chimera—a western way to the wealth of Asia.

In three voyages between 1576 and 1578, Martin Frobisher searched in vain for a Northwest Passage north of Labrador. Collecting pyrite samples, he returned to England in the mistaken belief that he had discovered gold reserves. In 1610 Henry Hudson—who had sailed up the Hudson River the previous year—set off to the bay that would bear his name, only to become trapped in ice. After the spring thaw failed to free the ship, the crew mutinied and set Hudson adrift. Robert Bylot, one of the few survivors, returned to Hudson Bay in 1612 to prove that a ship could survive an arctic winter. Bylot, in turn, recruited William Baffin, a navigator and surveyor of unsurpassed genius. In 1615–1616 Bylot and Baffin, backed by the Moscovy Company, pressed as far as 77° north, traveling through the Foxe Channel and Davis Strait, before being forced back by ice.

Frustrated in the search for a way around America, English colonists in seventeenth-century Virginia continued the search for a way across it, finding only a mountain barrier and an incalculably vast breadth of land. The foundation of a colony in Massachusetts in 1620 has undeserved renown as marking a new beginning in the history of the hemisphere, but it did mark a turning point in one respect: future English exploration of America would be more for the continent’s own sake than in pursuit of a route to Asia. The American dream succeeded dreams of the "gorgeous East."

Felipe Fernández-Armesto is the William P. Reynolds Professor of History at the University of Notre Dame after teaching at Tufts University, the University of London, and Oxford University. Among his publications, those of particular relevance to this topic include Millennium: A History of Our Last Thousand Years (1995), Pathfinders: A Global History of Exploration (2006), and 1492: The Year the World Began (2009).

Benjamin Sacks is a doctoral candidate in history and history of science at Princeton University and a 2009 national recipient of the Beinecke Graduate Scholarship. He is the book reviews editor for New Global Studies and chief editor emeritus of the Tufts Historical Review.