Over 40,000 tonnes of recycled wood chip has been burning all week at Rio Recycling in Orsett, Essex.
The wood chips were stored in a quarry that was 24m deep and 183m wide, and is being allowed to burn out by Essex Fire and Rescue.
It is not known what caused the blaze.
Essex Fire and Rescue divisional officer Ray Skinner said: “The wood appears to be clean and is not as yet burning pollutants into the atmosphere.
“We will keep this under constant review in conjunction with our colleagues from the Environment Agency.”