No one is quite sure when or how the coal fire in Liu Huangou first started - but it was certainly more than 20 years ago and maybe as long as 40.
Authorities in Xinjiang in north-west China are preparing to try and put out a massive underground coal fire that has been burning there for more than 20 years.
For decades, the blaze has been pumping sulphurous gas into the air causing serious pollution in and around the city of Urumqi.
Until now such fires have been considered all but impossible to extinguish.
But firefighters in Xinjiang using newly-developed techniques say they are confident of success, even if it will take them several more years to achieve.
Toxic gases
No one is quite sure when or how the coal fire in Liu Huangou first started - but it was certainly more than 20 years ago and maybe as long as 40.
In that time, millions of tonnes of coal have been consumed by the fire and huge amounts of toxic gases pumped into the air, fanning out across the nearby city of Urumqi.
Now the authorities have decided enough is enough, but putting out the fire will not be easy.
It will take at least four years and more than US $10m.
Pumping in mud
First, advanced sensoring devices will be used to detect where the underground fires are at their most intense.
Then holes will be drilled down into the fire zones and millions of litres of water and liquified mud pumped in to smother the flames.
Finally the whole area will be covered over with thousands of tonnes of soil to prevent any oxygen from getting back in and rekindling the fires.
Liu Huangou is just one of a number of hugh coal fires raging deep underground in northern China.
The area has some of the biggest untapped coal reserves in the world, but driven by poverty, local farmers have over the years dug thousands of small mineshafts down into the coal seams.
Authorities say it is often accidents inside these small illegal mines that are to blame for starting the underground fires.