When 33-year-old Shi Haiqing answered the phone in the safety center at a coal mine in Taiyuan early on Tuesday, he thought the call might be from one of his colleagues reporting on work safety.

But the caller asked: "Can you tell me about the gas density at the position where I am standing now? Can you tell where I am at the moment?"

Shi felt the voice sounded familiar, but lost no time in replying. Taking a glance at the control panel, he said, "The gas density is 0.05 (percent per cubic meter)."