MIAOGOU VILLAGE, China - Like millions of other Chinese, Li Zhanjun lives in a dwelling that is fireproof, noise proof, warm in winter, cool in summer and the epitome of an eco-friendly design. Moreover, it's cheap.

Li lives in a cave.

About 20 million Chinese still reside in caves and dirt-covered dwellings on the Loess Plateau that straddles the middle and upper reaches of the Yellow River in China's northwest. Some of the caves have been passed down for generations, with hard-packed earthen walls, electrical wiring, piped-in plumbing and other modern conveniences, including cable television.

Longtime cave dwellers are often passionate about their way of life, saying they are shielded from the elements in a practical and efficient way, dwelling along hillsides and leaving valuable arable land in valleys for growing crops.

Researchers say economic necessity isn't the only reason so many Chinese continue to reside in caves. "People from abroad think people who live in caves are very poor. But our research shows that is not always the case," said Wang Jun, a researcher on caves at Xian University of Architecture and Technology.

Many simply have grown accustomed to a lifestyle that dates back more than a millennium. Caves also have a revolutionary luster. Mao Tse-tung, the revolutionary founder of modern China, lived in caves that still honeycomb this region after the Long March, plotting the drive to take over the country that succeeded in 1949.

Caves are easily excavated from the silty soil, requiring only picks and other digging implements. The earth is so hard-packed that caves don't need additional support to prevent collapse. Most have a stone facade, with large lattice windows framing a door and allowing light to pour in during the day.

Generally, the caves are shaped like loaves of bread, 10 to 13 feet wide and anywhere from 20 to 25 feet deep, with arched ceilings. Several caves dug next to each other can have connecting doors, providing for a larger overall dwelling, with flues allowing for ventilation from indoor cooking fires.

The caves provide a cool respite from intense summer heat, and a snug retreat for inhabitants as winter temperatures drop.

Wang Aifang recalled one day last winter when temperatures fell to minus 4 degrees. When she stoked up an indoor cooking furnace, the cave quickly warmed up. "Look how the warm air circulates from the stove, under the bed and out the flue," she said, feeding a fire in an earthen hearth with branches from dried sesame plants.

Her dirt cave had walls so hard-packed they appeared to be made of concrete. They were covered with white lime, except at lower levels, which she had covered in newsprint. "The houses in the city have to use heat. We don't," Wang Aifang said.

Wang, the architect, who is not related, said researchers had done tests on the caves, finding that even when outside temperatures fell to 3 degrees, indoor climate in the cave stood at about 54 degrees.

Just a few decades ago, up to 40 million Chinese lived in caves. Back then, many caves had small doors and windows, making them dark and dank. The number of cave dwellers has dropped as living standards improved in China, even as cave designs have gotten more comfortable, with bigger doors and windows.

Wang is trying to persuade authorities to promote cave living, designing greenhouse fronts that allow them to trap solar heat more efficiently in winter. He said caves are far more energy efficient than freestanding buildings. "If you cook just a little food over a fire, it heats up the whole cave. A house isn't so efficient," he said.

Cave designs have changed over the years. As rural incomes rise, cave dwellers bring more furniture home. They've built walls that go up to about 6 feet, allowing for wardrobes. Ventilation systems have improved, and about 80 percent have indoor plumbing.

They remain largely impervious to natural disaster. "They are safe in earthquakes. Only landslides can damage them," Wang said. "They can't burn down."

One of the biggest advantages remains economic, he added. "You can live there forever. You don't pay anybody."