"If approved, work could begin as early as 2015 or 2016," Wang Mengshu, a tunnel and railway expert at the Chinese Academy of Engineering who has worked on the plan since 2012, told China Daily on 14 February.

The project will cost an estimated CNY 220bn (USD 36bn), he said.

Work on a feasibility report could take two or three years, he said.

The 123km underwater tunnel will house a rail line connecting the port cities of Dalian in Liaoning province and Yantai in Shandong province, according to the plan. Its planned life span will be about 100 years.

It will far surpass the combined length of the world’s two current longest underwater tunnels — Japan’s Seikan Tunnel linking Honshu and Hokkaido, and the Channel Tunnel between Britain and France.

"Using the tunnel, it will take only 40 minutes to travel from Dalian to Yantai," Wang said.

At the moment it is a 1,400km drive or about eight hours by ferry.

Passenger vehicles can be loaded onto rail carriages and transported at about 220km an hour, massively shortening the travel time between the two cities, according to the plan.

Wang said the project will consist of two underwater tunnels, both about 10m in diameter, and a service tunnel for maintenance and emergency vehicles about 7m in diameter.

Authorities in Shandong and Liaoning are hoping the project will stimulate economic growth by connecting the northern area with the wealthy eastern coast.

The project has been proposed at the National People’s Congress and the Chinese People’s Political Consultative Conference every year since 2009.

The project was mentioned in Shandong’s regional plan approved by the State Council in 2011.

Research by Ludong University in Yantai showed daily traffic flow between Dalian and Yantai is expected to increase to more than 100,000 vehicles by 2015, which will pose a challenge for transport capacity.

Liu Zhongliang, a professor involved in the research, said the tunnel is expected to bring profits of CNY 20bn a year and boost tourism in surrounding regions.

"The project could pay for itself within 12 years," Wang said.

Besides the profits, the tunnel can reduce the huge oil consumption required every day for cars traveling back and forth on highways, he said.

He added the Bohai Sea tunnel is a critical part of the country’s 5,700km railway project to link the cities of Tongshan in Heilongjiang province and Sanya in Hainan province.

The Bohai tunnel will be built at least 30m below the seabed, which is mostly hard rock. Two major fault zones are in the region, posing a risk. The Tanlu, and Zhangjiakou Penglai faults have been sources of chronic seismic activity. The Tangshan earthquake in 1976 killed tens of thousands of people.

Matthias Loftsson, director of geology for Iceland’s Mann-vit, which has decades of experience in land and sub-sea tunnel design, engineering and consulting, highlighted safety concerns.

"In general, though, one can say tunnels are not unsafe in earthquake areas, all depending on the geology, tunnel depth and other local conditions," he said.

"However, excavation of a tunnel through active faults, where displacement can occur with a potential danger of flooding, would be of great concern and needs special attention," Loftsson said.

Liu Jie, director of the China Earthquake Networks Center, agreed that Northeast China is unstable, with earthquakes of less than magnitude-5 frequently occurring, especially after a massive magnitude-9 earthquake in Japan in 1999 caused large-scale movement of the lithosphere and increased tremours.