At least 1 ton of ancient coins have been unearthed at a construction site 100 kilometres northeast of Xi'an, capital of Northwest China's Shaanxi Province.

Xinhua News Agency reported yesterday that the ancient coins dated back about 900 years to the Northern Song Dynasty (960-1127).

Cultural relics workers carried the coins away in 20 bags.

"The ancient coins are at Pucheng County Museum awaiting identification," said a staff member of Pucheng county's cultural relics bureau, who did not want to be identified.

The coins were found in a brick cellar about 6 or 7 metres underground when an excavator was working on the site on Sunday.

A witness said the cellar was full of rusty coins, some tied together with rotten leather strips. Some coins have been confirmed to belong to the Northern Song Dynasty while others were not identified because of erosion. The owner of the coins is a mystery.

Huge quantities of Northern Song Dynasty coins have been unearthed before. In the 1950s, 110 tons of coins were found in Huangshi of Hubei Province in Central China.

"That dynasty is regarded as one of the most prosperous periods in ancient China and a large number of coins were cast at that time," Zhou Weirong, an expert with the China Numismatic Museum, told China Daily.

Historical data shows that the number of coins cast each year in the peak period was 7 million strings each with 1,000 coins.

Zhou added it was interesting to study why so many copper coins were found in Shaanxi.

"The then governments banned border towns from using copper coins in case the metal, which was very precious at that time, fell in the hands of hostile countries. Shaanxi was located right on the border then," he said.