A gold seal of Zhang Xianzhong excavated from the Jiangkou site. [Photo by LIU LANYING/YANG ZHENGLAN/FOR CHINA DAILY]

In 2011, workers found many gold and silver items and coins while digging sand for construction purposes in the Pengshan-district section of the Minjiang in Pengshan's Jiangkou town.

The discovery enticed treasure hunters.

In 2016, police found 10 gangs of illegal relics excavators and nine illegal relics-trading networks had sold over 300 million yuan ($42 million) worth of artifacts from the site.

This pushed archaeologists to undertake Sichuan's first underwater archaeological expedition, says Liu Zhiyan, an archaeologist with the Sichuan Provincial Cultural Relics and Archaeology Research Institute.

Over 30,000 relics have been recovered, including gold, silver, bronze and iron items that date back about 400 years.

"The gold and silver coins we found have words related to Zhang Xianzhong," Liu says.

More than 1,000 artifacts, including Zhang's gold seal, are directly related to the Daxi Kingdom he founded after the Ming collapsed, institute president Gao Dalun says.

This provides evidence that the Pengshan section of the Minjiang was where Zhang's fleet was attacked and destroyed in 1646.

A museum will be built on the excavation site.