Construction of China's first underwater railway tunnel was completed Saturday in south China, which allows trains to operate at the world's top speed under the water.

The project, or the Shiziyang Tunnel, crossed the Pearl River estuary in south China's Guangdong Province with a length of 10.8 kilometers. It is designed for trains travelling at 350 kilometers per hour, the highest of all underwater tunnels worldwide.

The 10.8-kilometer tunnel, which is also the country's longest, is a key part of a 140-kilometer high-speed rail link that connects Guangzhou, the capital of China's southern economic powerhouse Guangdong, with the city of Shenzhen, also in Guangdong, and Hong Kong.

Liu Guangjun, project manager with the Shiziyang Tunnel, said large shielding machines had been used in digging of the tunnel at 60 meters underwater.

Construction of the tunnel started in November 2007, and the Guangzhou-Shenzhen-Hong Kong express rail link is scheduled to put into operation in 2012, which would slash travel time between Guangzhou and Hong Kong to 40 minutes from the current two hours.

The express is also expected to join with the country's express railway network and take passengers only eight hours from Hong Kong to Beijing.