This is the tale of an ordinary construction worker in Beijing who accidentally made a significant archaeological find 15 years ago but did not know that until very recently.

He discovered the ruins of an ancient water gate at the base of the city wall of the Jin Dynasty (1115-1234) in 1990.

It was soon listed as one of the national top 10 archaeological findings.

But 38-year-old Xu Jiandong only found that out a couple of weeks ago.

Xu told China Daily yesterday that he was engaged in a real estate construction project in Fengtai District in 1990.

"My job at that time was to excavate earth for the foundations. One afternoon, while driving a bulldozer, I dug out several wooden stakes and flagstones about 7 to 8 metres underground.

"The stakes were placed in two orderly rows and I found traditional designs on the flagstones. I thought they might be cultural relics so I stopped digging," said Xu.

He immediately called the Municipal Cultural Heritage Bureau .

"Then a group of officials from the bureau came to the spot and asked all the workers to stop digging," Xu recalled.

Several days later, Xu and his team were told by their boss to move out of the construction site.

"Nobody told me what was going on and I did not ask," said Xu. "I thought our work was finished as my team was only engaged in digging the foundations."

The young worker did not dream that his small findings would lead to a great archaeological discovery.

While talking to his nephew two weeks ago, Xu learnt what had happened all those years before.

Xu's nephew is a clerk with the Municipal Cultural Heritage Bureau.

Qi Xin, a renowned expert in the Jin Dynasty history research, said many historical documents recorded that as the capital city of the Jin Dynasty, the old city of Beijing had a very complicated and scientific drainage system.

"But history experts had not found out where the general drainhole was located before the discovery of the water gate," said Qi.

"Then the water gate was identified as the general drainhole of Beijing then, that drains water from inside the city wall to the outside," said Qi.

In order to protect the historical ruins, the municipal government injected more than 5 million yuan (US$600,000) into building a museum over the water gate.

Yesterday Xu was made an honorary member of the museum.

"If the construction worker had not had any sense about heritage protection, he would have probably continued digging and destroyed the ruins totally," said Qi.