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A major construction project has unearthed a piece of history and thrown light on engineering expertise dating back millennia.

Twenty ancient rock pillar bases, probably used to support a dam, have emerged from the depths near the Precious Bottleneck Channel in the wake of large-scale maintenance of the 2,000-year-old Dujiangyan Irrigation Project.

The project had been suspended for 11 years but was relaunched on Dec 10.

About 40 workers spent hours damming the Minjiang, a tributary of the upper Yangtze River, in Sichuan province.

Of course they had access to modern machinery but some of the methods they used harked back to earlier times.

One of the traditional methods they used was to put stones in bamboo cages, and sandbags around rafts, each made of tree trunks. This ensured an effective dam, much as it did thousands of years ago.

After the river was dammed, its bed emerged, allowing workers to clear the waterway.

But when the water level dropped, 20 ancient round rocks and boulders were found near the channel.

"The boulders might be building materials for maintaining the levee of the project, while the round stones might be the 'bedders' for increasing the stress area of the pillars of large buildings, which had been washed into the river by floods," said Fu Hao, deputy director of the Dujiangyan cultural heritage bureau.

Experts say the site has many submerged relics.

In previous centuries, the Chengdu Plain, now one of China's most important agricultural regions, suffered from incessant flooding of the Minjiang in the summer, and withered with drought in the winter.

（中國日報網英語點津 Helen 編輯）

About the broadcaster:

Anne Ruisi is an editor at China Daily online with more than 30 years of experience as a newspaper editor and reporter. She has worked at newspapers in the U.S., including The Birmingham News in Alabama and City Newspaper of Rochester, N.Y.