A Queensland underground coal mine has been on fire for five days and until all access to the surface is sealed, the inferno will continue.
Workers at the Grosvenor Coal Mine near Moranbah in Central Queensland were safely evacuated after a spark ignited methane gas on Saturday morning.
It's the same mine where an underground explosion in 2020 seriously injured five workers, forcing its closure until February 2022.
The mine on the town's outskirts employs about 1,400 people.
Sealing the mine is the only way to stop the fire, which is burning up to half a kilometre underground.
Remote control bulldozers have been brought in to push dirt towards the shafts.
What's burning?
The fire is fuelled by methane gas and coal.
Every year, millions of tonnes of coal are mined and taken to the surface by conveyor belt.
In longwall mining, coal is often left behind in what is called the goaf area.
Large volumes of methane can also be present in goaf areas that, given the right conditions, can be highly combustible.
While methane gas occurs naturally in coal seams, Grosvenor has higher levels than other coal deposits in Queensland's Bowen Basin.
How did the fire start?
This is still under investigation and may never be known.
After the 2020 Grosvenor disaster, Queensland's Coal Mine Board of Inquiry was unable to determine the ignition source.
Coal fires start when methane gas is ignited by a spark, and Stephen Smyth from the Mining and Energy Union said it would be difficult to determine the cause.
"It can be caused by friction ignition from rock-on-rock, a pick on a rock, it can be from various other sources of ignition," Mr Smyth said.
How do you seal a burning mine?
The only way to extinguish the fire is to starve it of oxygen in a strategic and controlled manner.
Remote-operated bulldozers push dirt and rocks into the shaft because it's too dangerous for people to be there.
In its latest statement, owner Anglo American said remote control technology was the key to the response with three dozers now assisting with the temporary sealing.
Industrial-level expanding foam can also be used to form a plug in the shaft, which is then capped with a steel plate that is covered with dirt.
A jet engine with an afterburner is also used to pump carbon dioxide, nitrogen, and water into the mine.
The 12-metre-long GAG unit is mounted on the back of a truck and emits carbon dioxide, nitrogen, and water vapour, which is pumped into the mine to starve the fire of the oxygen it needs to burn.
GAG stands for Górniczy Agregat Gaśniczy in Polish and this roughly translates to "mine fire suppression apparatus".
Why does it take so long?
The Grosvenor Coal Mine has a complex network of tunnels as deep as 500m underground.
At least nine openings need to be closed: six ventilation shafts, a conveyor belt that removes the coal from the mine, the main decline (called an intake airway where the fresh air comes in), and a return (the exhaust airway where the dirty air comes out).
These must be sealed in a systematic way to ensure gas isn't recirculated through the mine and pushed back over an open fire, which could result in an explosion.
It's a precision exercise that, if not done safely, can be deadly.
When a mine is on fire, the ventilation system is destroyed and there's no way to control the build-up of explosive gasses.
Queensland has a chequered history where lives have been lost while trying to seal burning coal mines.
In 1975, an explosion occurred in the underground workings of the Kianga No. 1 mine in Moura in central Queensland.
Thirteen men who were underground were killed while attempting to seal off heating coal.
It's 52 years since Box Flat Mine in Ipswich caught fire, trapping and killing 17 men. Another three died trying to seal the mine.
These disasters led to the formation of today's safety and gas detection systems and, decades later, the Mining and Energy Union is confident in the re-sealing plan.
"There's no doubt some of the best-trained people in the world are on site," Mr Smyth said.
"They know what they're doing, and they've had a practice over recent years sadly with North Goonyella [in 2018] and Grosvenor [in 2020]."
Is it working?
Anglo American said all work was proceeding safely and to plan.
"The sealing efforts mean the amount of oxygen available to the underground fire has greatly reduced," the company said.
"The QMRS mobile extinguisher unit (GAG) continues to help in this effort. As a result, smoke continues to reduce."
There is additional air quality monitoring equipment around the Moranbah township, which has a population of about 9,500.
"All monitors are reporting readings within acceptable limits, indicating no impact on community health from smoke or airborne contaminants," the company said.
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