The ruins were found in a major reservoir on the border shared by China and the Democratic People's Republic of Korea (DPRK). The tombs were found 20 kilometers away.

The ruins were spotted during repair work on the Yunfeng Reservoir situated in the outlying mountainous regions near the SINO-DPRK border, said Zhang Fuyou, Chairman of the Mount Changbai Cultural Society of Jilin Province in northeast China.

As a leader of a three-member research team, Zhang has just concluded a month-long inspection tour of the 1950s reservoir, also a tributary of the Yalu River flowing along the SINO-DPRK border. The water level in the reservoir was lowered by a depth of 41.13 meters to facilitate the repairing efforts.

The ancient city ruins, now covered by thick mud, are in a square formation and have a city wall 1.5 meters tall and four meters wide. There is also evidence of a moat around the city wall.

The 180-meter-long western side and a six-meter-wide gate on the western side are still visible, along with the 220-m-long northern side. The eastern side was buried by housing foundations built in later years and the southern side was destroyed by water from a local river.

A dozen tombs were also found inside the city ruins and in an area to the north.

Judging from the structure and construction style, the city ruins might have been built during the ancient Han Dynasty, acknowledged Zhang, who said this could not be confirmed until further excavation into the ruins had been carried out.

The team also found a sprawling tomb stretch, with 2,360 individual tombs in all, in the same reservoir but some 20 kilometers away from the location of the ancient city ruins. The massive tombs are believed to belong to the ancient Koguryo kingdom, which existed from the first to the fifth centuries AD.

The reservoir is proximate to Ji'an, a Chinese city on the SINO-DPRK border that drew global attention in July 2004 when it was included on the World Heritage List for being home to an even larger number of tombs, over 6,000, from the ancient Koguryo kingdom.