An ambitious plan to build the world's longest underwater tunnel, linking the northeastern coastal provinces of Shandong and Liaoning, has been proposed by a leading engineer, it was reported yesterday. He Yishou, an undersea tunnelling expert, said his proposed project would dramatically change the relationship between northeastern China and the rest of the country, particularly in transport and communications, as well as its links with other countries, the Qingdao News Web site reported. The 67-year-old professor said building a tunnel across the mouth of Bohai Bay - a distance of 147km - would be more technically demanding than the Three Gorges Dam, but would cost less. The professor has spent 10 years researching his idea without any public or private assistance. A railway ferry project, linking the coastal city of Dalian in the south of the Liaodong peninsula and Yantai city in the north of Shandong province, is already in the pipeline. The Yellow River runs into Bohai Bay, indenting the coast of northeastern China. It is bordered by Shandong, Hebei and Liaoning provinces, and the Tianjin municipality. According to the People's Daily, China's largest offshore oilfield, with proven reserves of 600 million tonnes, was discovered in the Bohai Sea in 2000. It is the second-largest oilfield after the Daqing oilfield. Last October, China National Offshore Oil Corporation, China's third-largest oil company, agreed with Texaco China to jointly develop an oilfield in Bohai Bay with oil and gas reserves estimated at 210 million cubic metres, according to the People's Daily. The world's longest undersea tunnel is the 54km Sei-kan tunnel in Japan.