The Land Cruiser rattles and bumps down a stripe of rutted dirt carving through the brush in this remote corner of southern Angola. Half a mile to the west, the tranquil blue Atlantic glimmers in the African sun. To the east, miles of spiky desert grass fade away to a range of sere mountains. The last village lies miles behind us, the next miles ahead.

In the front seat, Alain Gachet, a plump, boyish 58-year-old, his thick crest of silver hair crammed under a leather Indiana Jones hat, is focused intently on the laptop balanced on his knees. The computer is plugged into a tiny GPS unit set on the dashboard. On the screen, a thin yellow line tracking our progress creeps forward over a map stippled with thousands of differently colored squares.

"Stop here!" Gachet cries suddenly.

The driver brakes in the middle of the track. By the time three South African drillers and I extricate ourselves from the cramped vehicle, Gachet has bounded out, scrambled over a hillock and found a low, clear patch of sandy yellow soil.

"Right here, Freddy," Gachet requests in French-accented English.

Freddy Chambers, the beefy lead driller whose thick salt-and-pepper hair and mustache lend him a passing resemblance to Saddam Hussein, drives a shovel into the earth. Gachet practically vibrates with excitement as he watches. About 2 feet down, muddy gray water starts bubbling into the hole. Both men's faces split into grins.

Gachet fills an empty juice bottle with the cloudy liquid, strains it through a portable filter and drinks. "Fresh water," he says and bursts out laughing.

It's an extraordinary find, not only because the area is so dry, but because underground water this close to the sea would normally be too salty to drink. Gachet knew there would be fresh water in this spot, though; messages from outer space told him so.

Finding more fresh water is one of the paramount challenges of the 21st century. Nearly one-third of the human race lacks reliable access to clean water, according to the International Water Management Institute. Some 3 million people — most of them children — die every year from diseases spread by contaminated water. A 2007 report by the U.N. Environment Program predicts that by 2025, if population growth and environmental degradation continue apace, 1.8 billion people will live in countries with "absolute water scarcity."

A former oil industry geologist, Gachet has developed a path-breaking, high-tech system that could help slake that growing thirst. The key: using satellites high above the Earth's surface to see what's underneath it.

By combining terabytes of space-based photographic imagery, ground-penetrating radar and topographic data — much of which has only recently become available — Gachet creates multispectral maps that are proving excellent guides for finding undiscovered underground aquatic resources. At the height of the Darfur crisis, the United Nations called on Gachet to help find sustainable locations for camps in Chad that now house thousands of refugees. It was the first time such technology had been used in a humanitarian emergency. "Gachet's work was an extremely important contribution at a time when it was not sure that [the U.N.] would be able to provide water for the long term for all refugees," says Marc-Andre Bunzli, a former U.N. official who worked with Gachet in Chad. Since then, Gachet has located water in Darfur itself, as well as in parts of Afghanistan, Iraq and Eritrea.

I joined Gachet last summer for the kickoff of a new project. Joint Aid Management, a South Africa-based humanitarian group, recently began a campaign to provide food and water to some 450 schools scattered over hundreds of miles in war-blasted Angola. Because it costs an average of $10,000 to bore a hole for a well, the group has a major incentive to increase its hit rate. Joint Aid Management brought in Gachet to help them figure out where to dig.

"It's the first time I'm working not in a war zone but in a reconstruction area," Gachet told me when I first met him in JAM's compound, a fenced-off patch of desert full of trucks, housing trailers and a small processed food factory on the outskirts of Benguela, a coastal city in southern Angola. "Emergency situations are very frustrating. You maintain people in camps, but you don't deal with sustainable development. It's crazy, because you create a generation of beggars. Here, I hope the wells can bring prosperity and stability."

This beleaguered southern African nation can certainly use both. Colonized by Portugal for centuries, Angola won independence in 1975 after a lengthy guerrilla struggle — only to plunge almost immediately into an even more devastating civil war. The two main factions became Cold War cat's-paws, with the Soviet Union and Cuba arming the governing Movimento Popular de Libertacao de Angola, and the U.S. and South Africa backing the rebel Uniao Nacional para a Independencia Total de Angola. The fighting ground on for 27 years, until the MPLA finally beat UNITA down. The war left as many as 1.5 million Angolans dead, factories and cities in ruins, and roads and farms infested with landmines.

Today, seven years after the shooting stopped, Angola is still pretty much a basket case. The United Nations ranks it as one of the world's poorest countries despite its enormous natural resources. Twice the size of Texas, Angola is rich in diamonds, gold and other minerals, not to mention enormous oil reserves that are only beginning to be seriously tapped. New five-star hotels are surging up from the potholed streets of the capital, Luanda, and a tiny elite with connections to President Jose Eduardo dos Santos is reportedly pocketing fortunes in petro-dollars. But there's not much sign of that new wealth elsewhere. An estimated 70 percent of the nation's 13 million inhabitants live on less than one U.S. dollar a day; 35 percent are malnourished. A chronic lack of basic sanitation and health care help give Angola the world's second-highest infant-mortality rate. Those who survive can expect to die before reaching the age of 42.

Lack of clean water is one of the key factors driving those appalling statistics. One day, I visited a children's hospital in Cubal, a small town in the grasslands southeast of Benguela. Cubal consists of a few streets of government offices and shops housed in low cement buildings, surrounded by acres of mud-brick huts roofed with thatch or corrugated tin held down with rocks.

In the spartan hospital, dozens of scrawny, undersized children sprawl listlessly on thin mattresses or lie in the arms of their stoic mothers. One or two cry insistently, but quietly — they don't have the strength to scream. Many have the distended bellies and open sores that indicate extreme protein deficiency. Nearly all have severe diarrhea, an ailment that's typically caused by unclean water and can be fatal if left untreated. Every now and then, outbreaks of water-borne cholera bring in even sicker kids.

How big a problem is water? "Grandissimo," says Sister Milagros Moreno, the redoubtable Spanish nun who has worked in this church-run hospital for 18 years. "Most people get their water from the river. It's very contaminated. Everyone washes their clothes and bathes in it. But people don't have enough wood or gas to boil the water. Wells would be a huge help."

Gachet has been investigating what lies beneath African soil almost his entire life. He was born in northern Madagascar in 1951, the son of French colonial civil servants. His father, a botanist, started taking him on treks into the rainforest when he was 4 years old. That's when Gachet fell in love with rocks. Prehistoric fossils were everywhere. He was fascinated by the story of how the continents had split apart eons ago, leaving the history of their union inscribed in layers of subterranean stone.

As an adult, naturally, he became a geologist. He worked for ELF, the French oil giant, for two decades, helping to find new oil and gas fields from Gabon to Holland's portion of the North Sea. "I've always been involved in exploration," he says. "I tell people where the wealth is, how deep down and how they can reach it by drilling."

But he grew disillusioned working in The Republic of Congo during the civil war of the 1990s. "I was the one who had to co-sign our checks to the government. It was clear the money was going to buy weapons," Gachet says. "I was losing the pride of working for this company."

So he set up his own, which he eventually dubbed Radar Technologies International. In 1996, Gachet was contacted by a mining outfit that wanted to locate the source of the gold their Pygmy workers in The Republic of Congo kept finding in rivers. There were few maps of the area, and aerial photographs were no help; the rainforest canopy was too dense.

Gachet turned to two new pieces of technology. Using an early GPS unit, he followed the Pygmies into the jungle, marking the spot in each river where they pulled out nuggets. Then he bought newly available radar images of the area taken by the American space shuttle. The radar could penetrate clouds and jungle to give Gachet a rough sense of the shape and texture of the land beneath — big clues to its underlying geology. By overlaying those images onto his GPS data points, he was able to locate the gold's source. "I had the illumination that I was in front of a completely new way to explore the planet," he says.

That "new way" is known as remote sensing — the use of imagery collected from space to find things on the ground. Gachet has integrated a suite of such technologies into his exploratory work. C-band radar used by satellites maintained by Canada and the European Space Agency penetrates the ground to a depth of about 50 centimeters. Japan's JERS-1 satellite provides L-band radar, which goes down as far as 18 meters. NASA's Landsat satellites record images of the Earth using eight different wavelengths, from infrared to visible light. The most recent addition to this arsenal of space-based imagery became available in 2004, when NASA released topographical data gathered by the space shuttle. That has enabled researchers for the first time to create 3-D views of any area on the planet. "It's a fantastic gift from the United States to the world," Gachet says.

Using these technologies, Gachet continues to find oil, diamonds and other subterranean treasures for big corporations. In 2002, while studying radar images of the Libyan desert for Shell, he noticed evidence of huge amounts of moisture underground. After some research, he realized he was looking at leaks from Libya's "Man Made River." One of President Muammar Qaddafi's proudest achievements, this colossal underground pipeline carries water from an aquifer under the Sahara to the desert nation's coastal cities. "Billions of cubic meters of water were being lost into the sand," Gachet recalls. He passed on his findings to the Libyan government and a few months later, was surprised to find himself giving a personal presentation to Qaddafi. Gachet was hoping to win a contract to monitor the pipeline. He never heard back about that — but he did hear from friends that a furious Qaddafi had executed the engineers in charge of the project. (The Libyan embassy would not comment when I asked about this incident.)

That was the beginning of a new chapter for Gachet. "I thought, 'You are finding leaks,'" he says. "'That means you can find groundwater that no one else can see.'" He began building a system, dubbed WATEX, to find water by remote sensing.