IT DOESN'T TAKE LONG TO REACH the thin air of the Andes from Quito, because you are already in the thin air of the Andes. Ecuador's capital is more than 9,000 feet above sea level, and if you drive east, toward Papallacta Pass, you enter a series of valleys whose stark grandeur makes you feel like you're inside an Ansel Adams photograph, albeit one that features the occasional llama.

Eventually, the road descends in ill-mannered serpentines toward Sucumbíos province, its capital of Lago Agrio, and the Amazon basin. As the terrain flattens, the scenery changes and the Andean cloudforest morphs into a steamy infection of stores, cattle, farms, and people. Curving along the highway is a thick pipeline filled with crude oil. It's used as an elevated walkway by children and adults who don't want to get stuck in the black mud below. They walk, literally and magically, on a path of oil.

This is the Trans-Ecuadorean Pipeline System, known by its Spanish initials, SOTE. More than 300 miles long, SOTE was built in the early 1970s; in 2004 it gained a twin, the Heavy Crude Pipeline (OCP), which doubled the country's capacity to transport oil over the Andes to the Pacific port of Esmeraldas, and from there to the United States. Ecuador now produces nearly 500,000 barrels a day. Every 24 hours, 300,000 of those are shipped to El Norte, making it Latin America's third-largest oil supplier to the U.S., behind Mexico and Venezuela.

Feeding this aorta is a web of smaller pipelines spread over the humid flatlands. The feeder pipes aren't buried or routed away from roads and people, as they would be in a wealthier nation. They rest on rickety pylons one or two feet high and just a few feet, or sometimes inches, from the road. If you swerve into one to avoid a pothole or lose control of your vehicle because you are drunk, you will create an oil spill. It happens all the time.

Thirty-five years ago, none of this existed. The road from Quito didn't extend beyond the Andes, and because there was no road, the 20th century hadn't penetrated the Oriente. It was a tangled expanse of jungle inhabited by indigenous Indians—Cofán, Huaorani, Secoya, Siona, and Quechua—numbering up to 20,000 or so at most. The territory was largely left alone until Texaco was granted the Oriente concession in 1964 and found oil in 1967.

What happened next is an old story. To extract and ship the dark product, hundreds of miles of roads were built, along with hundreds of wells, processing stations, and waste pits. A Wild West–style boomtown sprung up in the 1970s, replete with bars, prostitutes, and fistfights. Officially called Nueva Loja, the town quickly became known by its nickname, Lago Agrio—Spanish for "Sour Lake," the name of the Texas town where Texaco was founded in the early 1900s. Settlers poured in. To prevent neighboring Colombia from snatching the sparsely populated Oriente and to relieve overpopulation elsewhere in Ecuador, the government offered free land to anyone who'd clear the thick jungle and start farming. Their numbers already decimated over the centuries by Western diseases such as smallpox, the Cofán and other tribes were pushed farther into the disappearing jungle. Today, perhaps 1,000 Cofán remain.

On paper at least, Ecuador's government was the controlling authority in this enterprise. Texaco entered a joint-venture accord with a newly created state company called CEPE (now known as Petroecuador) to co-manage the extraction. But the Ecuadoreans had little expertise in oil drilling. When crude began to flow, the country's oil industry, steered by Texaco, all but regulated itself.

Texaco took out 1.5 billion barrels over a 20-year span, bringing the company profits that nobody seems to agree on. The San Francisco-based environmental group Amazon Watch, which works with the Frente to publicize the damage in the Oriente, claims Texaco netted more than $30 billion in profits. Chevron insists that the joint venture grossed $25 billion but that only $490 million went to the American company after royalties and taxes. Whatever the numbers, Ecuador's national government embarked on an unchecked spending spree that far exceeded its revenues. By the end of the 1990s, the country was saddled with $16 billion in debt; about 70 percent of Ecuadorean children still live below the poverty line, according to UNICEF.

The long regulatory leash didn't work out so well. To industry critics, Ecuador is Exhibit A of what happens when an oil company operates unfettered in a remote corner of the world. During oil extraction, water is often pumped deep into underground reservoirs to force out the crude, and when it comes up, so does "formation water"—a cocktail of leftover oil, metals, and water that can include benzene, chromium-6, and mercury. The plaintiffs charge that, instead of reinjecting this wastewater deep underground or removing the contaminants—standard practices now, and often done back then as well—Texaco dumped it into tributaries of the Amazon and hundreds of unlined pits. Without naming specific sources, the suit alleges that "close to 83 percent of the population has suffered diseases attributable to contamination... the more affected population is that of children under 14 years of age."

To buttress the suit's accusations, Donziger and others point to a number of studies, beginning with Amazon Crude, a 1991 environmental overview compiled by City University of New York law professor Judith Kimerling and published by the Natural Resources Defense Council; and the 1999 "Yana Curi" ("Black Gold") report, conducted by two Spanish medical researchers in cooperation with the Department of Tropical Medicine and Hygiene at the University of London. That study focused on the town of San Carlos, a hub of Oriente drilling activity, finding 2.3 times more cancers in men than the average in Quito, along with lymphoma rates in women that were 6.7 times higher than those in Quito. But San Carlos has a population of only 1,000, according to the study; that's just a small sample of the Oriente's 300,000 estimated residents.

Though there's a shortage of hard data, journalists regularly emerge from the Oriente with horror stories about sick people and livestock, and about polluted land, swamps, and rivers. In 2003, the Frente hired an American engineering consultant named David Russell to study the potential cost of a thorough cleanup. His Georgia-based firm, Global Environmental Operations, estimated a final price tag of $6 billion, a figure that Chevron disputes. Company spokesman Chris Gidez—a public-relations expert at Hill & Knowlton—calls this number "a wild claim." Even Donziger is speculating when he says it could cost as much as $10 billion—or far less. The truth is, nobody knows for sure.