Dr. Robert Zeigler, an eminent American botanist, flew to Saudi Arabia in March for a series of high-level discussions about the future of the kingdom's food supply. Saudi leaders were frightened: heavily dependent on imports, they had seen the price of rice and wheat, their dietary staples, fluctuate violently on the world market over the previous three years, at one point doubling in just a few months. The Saudis, rich in oil money but poor in arable land, were groping for a strategy to ensure that they could continue to meet the appetites of a growing population, and they wanted Zeigler's expertise.

There are basically two ways to increase the supply of food: find new fields to plant or invent ways to multiply what existing ones yield. Zeigler runs the International Rice Research Institute, which is devoted to the latter course, employing science to expand the size of harvests. During the so-called Green Revolution of the 1960s, the institute's laboratory developed "miracle rice," a high-yielding strain that has been credited with saving millions of people from famine. Zeigler went to Saudi Arabia hoping that the wealthy kingdom might offer money for the basic research that leads to such technological breakthroughs. Instead, to his surprise, he discovered that the Saudis wanted to attack the problem from the opposite direction. They were looking for land.

In a series of meetings, Saudi government officials, bankers and agribusiness executives told an institute delegation led by Zeigler that they intended to spend billions of dollars to establish plantations to produce rice and other staple crops in African nations like Mali, Senegal, Sudan and Ethiopia. "They laid out this incredible plan," Zeigler recalled. He was flabbergasted, not only by the scale of the projects but also by the audacity of their setting. Africa, the world's most famished continent, can't currently feed itself, let alone foreign markets.

The American scientist was catching a glimpse of an emerging test of the world's food resources, one that has begun to take shape over the last year, largely outside the bounds of international scrutiny. A variety of factors -- some transitory, like the spike in food prices, and others intractable, like global population growth and water scarcity -- have created a market for farmland, as rich but resource-deprived nations in the Middle East, Asia and elsewhere seek to outsource their food production to places where fields are cheap and abundant. Because much of the world's arable land is already in use -- almost 90 percent, according to one estimate, if you take out forests and fragile ecosystems -- the search has led to the countries least touched by development, in Africa. According to a recent study by the World Bank and the United Nations Food and Agriculture Organization, one of the earth's last large reserves of underused land is the billion-acre Guinea Savannah zone, a crescent-shaped swath that runs east across Africa all the way to Ethiopia, and southward to Congo and Angola.

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Investors who are taking part in the land rush say they are confronting a primal fear, a situation in which food is unavailable at any price. Over the 30 years between the mid-1970s and the middle of this decade, grain supplies soared and prices fell by about half, a steady trend that led many experts to believe that there was no limit to humanity's capacity to feed itself. But in 2006, the situation reversed, in concert with a wider commodities boom. Food prices increased slightly that year, rose by a quarter in 2007 and skyrocketed in 2008. Surplus-producing countries like Argentina and Vietnam, worried about feeding their own populations, placed restrictions on exports. American consumers, if they noticed the food crisis at all, saw it in modestly inflated supermarket bills, especially for meat and dairy products. But to many countries -- not just in the Middle East but also import-dependent nations like South Korea and Japan -- the specter of hyperinflation and hoarding presented an existential threat.