Seen from a plane, the car would have looked like a slow beetle creeping across an endless beach, the sun glinting off its polished black armor. In fact it was doing thirty miles per hour, the maximum safe speed on a road that had unexpected potholes and cracks. No one wanted to get a flat tire in the Sahara Desert.

The road led north from N'Djamena, capital city of Chad, through the desert toward Lake Chad, the biggest oasis in the Sahara. The landscape was a long, flat vista of sand and rock with a few pale yellow dried-up bushes and a random scatter of large and small stones, everything the same shade of mid-tan, as bleak as a moonscape.

The desert was unnervingly like outer space, Tamara Levit thought, with the car as a rocket ship. If anything went wrong with her space suit she could die. The comparison was fanciful and made her smile. All the same she glanced into the back of the car, where there were two reassuringly large plastic demijohns of water, enough to keep them all alive in an emergency until help arrived, probably.

The car was American. It was designed for difficult terrain, with high clearance and low gearing. It had tinted windows, and Tamara was wearing sunglasses, but even so the light glared off the concrete road and hurt her eyes.

All four people in the car wore shades. The driver, Ali, was a local man, born and raised here in Chad. In the city he wore blue jeans and a T-shirt, but today he had on a floor-length robe called a galabiya, with a loose cotton scarf wound around his head, traditional clothing for protection from the merciless sun.

Next to Ali in the front was an American soldier, Corporal Peter Ackerman. The rifle held loosely across his knees was a US Army standard-issue short-barreled lightweight carbine. He was about twenty years old, one of those young men who seemed to overflow with chirpy friendliness. To Tamara, who was almost thirty, he seemed ridiculously young to be carrying a lethal weapon. But he had no lack of confidence-one time he had even had the cheek to ask her for a date. "I like you, Pete, but you're much too young for me," she had said.

Beside Tamara in the rear seat was Tabdar "Tab" Sadoul, an attachŽ at the European Union mission in N'Djamena. Tab's glossy mid-brown hair was fashionably long, but otherwise he looked like an off-duty business executive, in khakis and a sky-blue button-down shirt, the sleeves rolled to show brown wrists.

She was attached to the American embassy in N'Djamena, and she wore her regular working clothes, a long-sleeved dress over trousers, with her dark hair tucked into a headscarf. It was a practical outfit that offended no one, and with her brown eyes and olive skin she did not even look like a foreigner. In a high-crime country such as Chad it was safer not to stand out, especially for a woman.

She was keeping an eye on the odometer. They had been on the road a couple of hours but now they were close to their destination. Tamara was tense about the meeting ahead. A lot hung on it, including her own career.

"Our cover story is a fact-finding mission," she said. "Do you know much about the lake?"

"Enough, I think," Tab said. "The Chari River rises in central Africa, runs eight hundred and seventy miles, and stops here. Lake Chad sustains several million people in four countries: Niger, Nigeria, Cameroon, and Chad. They're small farmers, graziers, and fishermen. Their favorite fish is the Nile perch, which can grow to six feet long and four hundred pounds."

Frenchmen speaking English always sounded as if they were trying to get you into bed, Tamara thought. Perhaps they always were. She said: "I guess they don't catch many Nile perch now that the water is so shallow."

"You're right. And the lake used to cover ten thousand square miles, but now it's only about five hundred. A lot of these people are on the edge of starvation."

"What do you think of the Chinese plan?"

"A canal one thousand five hundred miles long, bringing water from the river Congo? Chad's president is keen on it, not surprisingly. It might even happen-the Chinese do amazing things-but it won't be cheap, and it won't be soon."

China's investments in Africa were regarded, by Tamara's bosses in Washington and Tab's in Paris, with the same mixture of awestruck admiration and deep mistrust. Beijing spent billions, and got things done, but what were they really after?

Out of the corner of her eye Tamara saw a flash in the distance, a gleam as of sunlight on water. "Are we approaching the lake?" she asked Tab. "Or was that a mirage?"

"We must be close," he said.

"Look out for a turning on the left," she said to Ali, and then she repeated it in Arabic. Both Tamara and Tab were fluent in Arabic and French, the two main languages of Chad.

"Le voilˆ," Ali replied in French. Here it is.

The car slowed as it approached a junction marked only by a pile of stones.

They turned off the road onto a track across gravelly sand. In places it was hard to distinguish the track from the desert around it, but Ali seemed confident. In the distance Tamara glimpsed patches of green, smudged by heat haze, presumably trees and bushes growing by the water.

Beside the road Tamara saw the skeleton of a long-dead Peugeot pickup truck, a rusting body with no wheels or windows, and soon there were other signs of human habitation: a camel tied to a bush, a mongrel dog with a rat in its mouth, and a scatter of beer cans, bald tires, and ripped polythene.

They passed a vegetable patch, plants in neat straight lines being irrigated by a man with a watering can, then they came to a village, fifty or sixty houses spread randomly, with no pattern of streets. Most of the dwellings were traditional one-room huts, with circular mud-brick walls and tall pointed roofs of palm leaves. Ali drove at walking pace, threading the car between the houses, avoiding barefoot children and horned goats and outdoor cooking fires.

He stopped the car and said: "Nous sommes arrives." We have arrived.

Tamara said: "Pete, would you please put the carbine on the floor? We want to look like students of ecology."

"Sure thing, Ms. Levit." He put the gun by his feet, with its stock hidden under his seat.

Tab said: "This used to be a prosperous fishing village, but look how far away the water is now-a mile, at least."

The settlement was heartbreakingly poor, the poorest place Tamara had ever seen. It bordered a long, flat beach that had presumably been underwater once. Windmills that had pumped water to the fields now stood far from the lake, derelict, their sails turning pointlessly. A herd of skinny sheep grazed a patch of scrub, watched by a little girl with a stick in her hand. Tamara could see the lake glittering in the distance. Raffia palms and moshi bushes grew on the near shore. Low islets dotted the lake. Tamara knew that the larger islands served as hideouts for the terrorist gangs who plagued the inhabitants, stealing what little they had and beating any who tried to stop them. People who were already impoverished were made absolutely destitute.

Tab said: "What are those people doing in the lake, do you know?"

There were half a dozen women standing in the shallows, scooping the surface with bowls, and Tamara knew the answer to Tab's question. "They're skimming edible algae from the surface. We call it spirulina but their word is dihŽ. They filter it, then dry it in the sun."

"Have you tried it?"

She nodded. "It tastes awful but apparently it's nutritious. You can buy it in health food shops."

"I've never heard of it. It doesn't sound like the kind of thing that appeals to the French palate."

"You know it." Tamara opened the door and stepped out. Away from the car's air-conditioning, the atmosphere struck her like a burn. She pulled her scarf forward on her head to shade her face. Then she took a photo of the beach with her phone.

Tab got out of the car, putting a wide-brimmed straw hat on his head, and stood beside her. The hat did not suit him-in fact it looked a bit comical-but he did not seem to care. He was well-dressed but not vain. She liked that.

They both studied the village. Among the houses were cultivated plots striped with irrigation channels. The water had to be brought a long way, Tamara realized, and she felt depressingly sure that it was the women who carried it. A man in a galabiya seemed to be selling cigarettes, chatting amiably with the men, flirting a little with the women. Tamara recognized the white packet with the gold-colored sphinx head: it identified an Egyptian brand called Cleopatra, the most popular in Africa. The cigarettes were probably smuggled or stolen. Several motorcycles and motor scooters were parked outside the houses, and one very old Volkswagen Beetle. In this country the motorcycle was the most popular form of personal transport. Tamara took more pictures.

Perspiration trickled down her sides under her clothes. She wiped her forehead with the end of her cotton headscarf. Tab took out a red handkerchief with white spots and mopped under the collar of his button-down.

"Half these houses are unoccupied," Tab said.

Tamara looked more closely and saw that some of the buildings were decaying. There were holes in the palm-leaf roofs and some of the mud bricks were crumbling away.

"Huge numbers of people have left the area," Tab said. "I guess everyone who has somewhere to go has gone. But there are millions left behind. This whole place is a disaster area."

"And it's not just here, is it?" said Tamara. "This process, desertification at the southern edge of the Sahara, is happening all across Africa, from the Red Sea to the Atlantic Ocean."

"In French we call that region le Sahel."

"Same word in English, the Sahel."...