MORE than 80 members of the tiny, impoverished, Bolivian village of Lakaya are literally digging up their past. Using the outline of mysterious 800-year-old ridges and depressions as their guide, they are constructing, or to be more precise, reconstructing, 100 meter (328 foot) platforms of earth."We thought these ridges were just places for the children to play," says Bonifacia Quispe, an Aymara Indian woman wearing the traditional long plaits, bowler hat, and wide skirt. "Now we know better." Ms. Quispe is the president of a local Mother's Club, one of nearly 50 Indian groups from this 4,000 meter-high, poverty-stricken Andean region that have unlocked an ancient secret of their ancestors - how to grow bumper potato crops. "We're getting single potatoes up to 1.5 kilos [3.3 pounds] in weight," says Tomas Aranda, a village leader from Lakaya. "Before, our potatoes weighed only 60 grams [2.1 ounces]. Now one plant can produce 60 to 70 potatoes." Agronomists working with the villagers claim Lakaya now holds the world record for potato yields, an equivalent of 70 tons of potatoes per hectare (2.47 acres), compared with the 2.5 tons the villagers were getting five years ago. But Quispe, Mr. Aranda, and the villagers use no tractors, no artificial fertilizers, and no large influxes of capital - their technology is pure spades and picks. "In the last few years, our potatoes and other crops have been affected by frost and hail," Aranda says. "So we decided to build 'raised fields,' like our ancestors, who knew how to stop the weather ruining their crops." Aranda is one of 2,500 peasant farmers now using the raised fields, or Suka Kollus as they are known in Aymara. Raised fields consist of earth platforms, between 60 and 120 meters (197 to 416 feet) in length and between 3 and 6 meters (about 10 and 20 feet) in width. The earth is taken from canals separating the fields that are 60 centimeters-deep (about 2 1/2 feet). "During the day, the canals capture the fierce sunlight at this altitude, and keep the temperature of the Suka Kollus up during the freezing night," explains Oswaldo Rivera, the director of the Bolivian Institute of Archaeology, and a constantly joking friend and adviser to the villagers. "When the temperature drops suddenly, the waters in the canal form a type of capping mist over the fields, like a blanket," Mr. Rivera adds. "And water is drawn up into the soil platform, which heats the roots of the vegetables." Yet another advantage is that nitrogen-fixing bacteria form a sediment at the bottom of the canals, which can be scraped out in the dry season and used as a rich organic fertilizer. In 1978, Rivera and a United States academic, Alan Kolata, who is a professor of archaeology and anthropology at the University of Chicago, started to puzzle over the ridges and depressions that stretch like a huge corrugated tin roof over the Koani Pampa, a 35,000-acre area of the Bolivian altiplano. They were sure the ridges held the key to explaining the vast extent of the Tiwanaku (pronounced Tee-wah-NAH-koo) civilization, which reached its apogee from AD 450 to 800 before collapsing around 1150. At its peak, the Tiwanaku civilization probably dominated a California-size area of what is now western Bolivia, northern Chile, and southern Peru. TODAY the valley of Tiwanaku supports a population of only 7,000, whereas a millennium ago Prof. Kolata estimates that it supported more than 250,000 people, and many more beyond the valley. How could the area have been the breadbasket of the Andes, when modern agronomists had universally dismissed it as unproductive? Suspecting that the ridges held the answer, Kolata and Rivera eventually managed to persuade villagers to help them reconstruct a raised field. Their academic curiosity dovetailed with their desire to help the local communities, at least half of whose children suffer from malnutrition. "It took us seven years to gain the trust of the villagers," Kolata recalls, "but we eventually started in 1986 with a field in production." What helped to sway the villagers was a "fortunate but terrible" frost that year, which destroyed most of the crops they were growing on their traditional plots on the hillsides above the swampy, frost-ridden plains, where only sheep and llamas used to graze. The two pioneer communities using the raised-field system lost only 10 percent of their crops. "Our ancestors didn't know how to read and write," says Tomas Aranda, one of the villagers who had to be convinced, "but we know now that they really understood about the weather and water." The agricultural achievements of the Tiwanaku people are matched by similar advances in metallurgy, stone work, and hydrology, which promise to put the Tiwanaku civilization on the same level as the better-known Inca empire that followed it. "We've found a 1 kilometer-long sewer at Tiwanaku city [just a few miles west of Lakaya], which has a perfect 2 percent grading," Kolata says. "That's of sufficient standard for modern-day Boston." The discovery of the raised-field system also helped to convince archaeologists that Tiwanaku was not, as previously thought, just a ceremonial center, but the apex of a vast empire supported by the agricultural surplus from the Suka Kollus. TIWANAKU'S complex society - raised fields, sewage system, and all - collapsed , probably as a result of a prolonged 80-year drought. But for Rivera, Tiwanaku is still alive. He now plans to expand the raised-field system, which he calls the "only successful case of applied archaeology in Latin America. It's successful because the people who participate can eat the results." His other priority is to try to rehabilitate a huge spring- and river-fed canal system that fed the raised fields and helped to control flood damage. So far, two kilometers of canals, which were 8-to-20 meters wide and the "key to the functioning of the raised fields," have been dug up. He also wants to increase the area under cultivation from the current 240 acres to 5,000 acres in the next five years. That could mean a production of 40,000 tons of potatoes or about 6 percent of Bolivia's total production. It could also give the project, which initially received funding from the Inter-American Foundation and the Dutch government, some financial independence. Rivera is optimistic that the system could be applied in other poor areas of Bolivia - or even the world - provided there is a sufficient water supply from local springs or rivers. But even if the raised-field system cannot be exported, Rivera is convinced the project has already succeeded. "What we have here is a technology transfer in time, and not in space," he points out. "Bolivia has imported many new technologies from abroad, and many have been disasters," he adds. "This system has come from the villagers' ancestors, and that's given them a belief in their own capacity - and a real source of national pride."