When it comes to building infrastructure, China is among the fastest. However, the Dazhunshan Railway Tunnel, located in southwestern China’s Yunnan province, is famous for a different reason.

Started in 2008, construction of the bridge was originally scheduled to be finished in 2014. However, unexpected and complicated geographic conditions have dragged its pace down. With a total length of 14.5 kilometers, the tunnel has been described by some as the most challenging one ever built in China.

During its construction, intense pressure above the tunnel has often triggered floods and cave-ins, which can ruin engineers’ efforts in the blink of an eye. The overall volume of floods that have occurred is 150 million cubic meters, enough to fill Hangzhou's West Lake 15 times.

The work done to cross a water-rich fault called Yanziwo is a typical example of the difficulties workers have encountered. Compared to digging into a piece of tofu, the process of getting through the 156-meter-long fault took two years, and workers were forced to battle numerous mudslides. Engineers had to dig through 39 different kinds of rock, one of which reaches temperatures of 40 degrees Celsius; workers had to pause every two hours to cool down by sitting on ice cubes.

Dazhushan tunnel is a key connector of the Dali-Ruili Railway, also known as Darui Railway. Designed to run from Dali in central Yunnan to Ruili in southwestern Yunnan, Darui Railway is also an important component of the broader China-Myanmar railway project.

Construction of the tunnel is now entering its final stage. Jiang Dong, project manager from the China Rail First Group, expects Darui Railway to enter full service by 2021, as long as construction goes as planned from here on out. Once completed, it will take the train only seven minutes to run through the entire tunnel.