China has started construction on the world's first single-hole double-track high speed railway sea tunnel designed to allow trains to travel at a speed of up to 350 kilometers per hour in Shantou Bay, South China's Guangdong Province. The 163-kilometer-long railway, which includes a 9,781-meter-long tunnel, has officially entered the substantive construction stage, with an investment estimated to be at 26 billion yuan (3.7 billion U.S. dollars).