Deep beneath China’s seabed, workers just spent 21 days underwater while building the world’s longest undersea high-speed rail tunnel, linking the Zhoushan islands with China's rail network

China has completed a difficult stage of work on a massive railway tunnel being built beneath the seabed off Zhejiang province, with workers spending 21 days carrying out maintenance on a giant tunnel boring machine under extreme underwater pressure.

The operation involved replacing 104 cutting tools at the front of the machine. The tools help the machine break through rock and soil as it digs its way through the seabed.