Among the 20 primary and middle school students who had gathered for a free weekend English class recently in Xichang, Southwest China's Sichuan province, sat 63-year-old Hu Bigui.

"I'm learning English so I can find out if there are foreigners interested in unraveling the mystery of an ancient city hidden in Qionghai Lake," explains Hu, a retired teacher of the Xichang Railway Vocational School.

In 2008, it was widely reported that a city of the Han Dynasty (206 BC-AD 220) might lie submerged in Qionghai Lake, in Xichang, better known as the nation's satellite launch center.

Actually, it was Hu Bigui who started the story.

In May that year, Hu clicked on Google Earth and found a satellite photo of Qionghai. He found that one part of it looked as if something lay hidden in the waters.

Hu remembered reading about an ancient city that had mysteriously disappeared. He did some research and called the local media, triggering heated debate about the existence of such a city.

"There is a 98 percent possibility that an ancient city does exist in the lake," Hu says, adding that the city was mentioned in many ancient books as Qiongdu, a county established by Emperor Wudi (156-87 BC) of the Western Han Dynasty.

Liu Hong, curator of the Liangshan Prefecture Museum (Xichang is the capital of Liangshan prefecture), carried out field surveys with a team of archaeologists on May 7, 2008, after hearing the reports.

However, when they sent photos to the National Remote Sensing Center in Beijing, experts said the regular patterns seen on the Google photo may have just been farms.

Liu studied the area more thoroughly and found that it was the alluvial fan of the Ezhang River, formed after the Han Dynasty period. The nearby villages all came into existence after the Ming Dynasty (1368-1644).

Liu says if what was seen in the Google photo is indeed an ancient city, it would cover some 6 sq km, accommodating at least 200,000 people. However, there are no records of such a huge city in the area during the Han Dynasty.

But none of this convinced Hu, who decided to do his own exploration and spend some 3,000 yuan ($440) to invest in some Italian diving gear.

But owing to his age and limited diving skills, he could only dive to less than 3 m under water. He claims he once caught a glimpse of something like a city wall, but could not go any deeper to verify it.

The diving has also affected his hearing. Hu realized that he could never crack the mystery of the sunken city by himself, as Qionghai covers 30 sq km with an average depth of 14 m; at its deepest, it goes down more than 30 m.

He went online for help and established contact with a well-known diver, Geng Wei, of Yunnan province. Geng has dived in most lakes in Yunnan and helped discover a hitherto unknown city, dating back 1,800 years, lying under Fuxian Lake, outside Kunming.

Last May, Geng arrived in Xichang with four friends and advanced diving equipment. But they could not get a permit from the local authorities, as the staff were on holiday during the May Day break, and Geng could not wait.

Hu is looking forward to this April, when the water becomes warm enough for diving. He hopes Geng will be able to join the mission.

"Our exploration will reveal the truth; it is a golden chance to promote my hometown," Hu says.