Secret tunnels and unregistered workers: China's coal mine disaster is a reminder of darker days

In Shanxi, the province that sits at the heart of China's coal-mining industry, there's long been a saying: "Only go down a coal pit when you have no other way out."

For decades, life in these pits was intertwined with tragedy.

It became so common that it gave rise to other sayings: about how miners were "exchanging their lives for money" or "staking their lives for tomorrow" when they ventured into underground tunnels where they died from gas explosions, flooding and shaft collapses.

Over the past decade, safety reforms steadily erased the industry's deadly reputation, and those days were thought to be behind China - until 22 May, when a blast at the Liushenyu coal mine in Shanxi killed 82 people and injured more than 120 others.

China's worst coal mining disaster in more than 15 years happened as the country continues its ambitious pivot towards green energy - a reminder that it is still struggling to shake off its dependency on an industry that has proven dangerous so many times in the past.

"Everyone knew this was a high-methane mine," says Chen, a miner who previously worked at the Liushenyu coal mine for two years.

"My feeling is there must still be miners inside. The tunnels underground are complicated and criss-crossed. There are hidden mine faces."

With a mine like this, Chen says, "it was only a matter of time" until disaster struck.

Hopes of finding survivors have been all but extinguished at the Liushenyu coal mine.

"The explosion swept to the entrance and knocked all of us down. We could not see anyone; the dust was incredibly thick," a survivor later told China's state-run news outlet CCTV. "After running for more than 10 minutes, my consciousness blurred. I was terrified."

Authorities are yet to confirm the cause of the blast, but experts tell the BBC that such explosions typically happen when a build-up of methane gas or coal dust comes into contact with an ignition source.

And that even in inherently risky mine environments, human error most often proves to be the fatal factor: management failure, flawed safety systems and flouted protocols.

A properly designed coal mine is "fully capable of preventing an explosion through systematic safeguards," explains Hong Chen, a professor at Jiangnan University's Institute for National Security and Green Development.

"Based on the coal mine safety management and technical systems we have in place today, let me be very clear about this: this accident should not have happened."

Initial findings show Tongzhou Group, the company operating the privately owned coal mine, had committed "seri