At another site, Huang Zhixiu, 52, chops off a nearly 2-meter-long icicle, and suddenly ice splinters clatter around.

If any of them hit the face they can cause great damage.

Huang gasps while chopping the ice, saying that there was a time when his face was cut open and covered in blood.

Compared with ice-breaking on the cave walls, ice-breaking on the roof is even more dangerous. Another ice-breaking team led by Captain Li Huanqun is responsible for the task.

Their tool to clear the ice is an insulated rod.

"The top of the tunnel has high-voltage railway lines with voltage up to 27,000 volts, and we have to check them before each ice-breaking attempt to ensure safety," says Li Huanqun. "Although the current generation of insulated rods are much lighter than the old ones, holding a 7-meter-long pole for two hours is strenuous work."

For more than 30 years, the wall ice-breaking team and the rooftop ice-breaking team meet in this deep tunnel every day.