A diesel laundering plant, hidden in a remote farm building, has polluted an extensive area of marshland in south Armagh.
HM Revenue & Customs (HMRC) said it was capable of producing up to 1.75m litres of illicit fuel a year,
The plant in Silverbridge, which was capable of evading over £1m in tax has been dismantled.
HMRC has removed over 20,000 litres of toxic waste, the residue of the laundering process.
HMRC officers assisted by the Police Service of Northern Ireland (PSNI) searched a farm in the Silverbridge area, where they uncovered equipment and farm machinery being used to launder red diesel along with several vehicles for transporting the laundered fuel.
Underground slurry tanks were used to store the waste, which had leaked into a nearby river, with other waste dumped on nearby marshland.
Assistant director of HMRC specialist investigations Mike Connolly said those responsible had shown a total disregard for the local land and waterways.
"Indiscriminate dumping of the by-products from the laundering process can cause severe damage to the environment, as well as taxpayers and local ratepayers having to pay for the clean up and disposal costs.
"People need to be aware of the environmental and safety issues surrounding the laundering of fuel, which is often done in the midst of rural or farming communities.
Mr Connolly said HMRC takes the "dangerous" and "illegal" practice of laundering fuel very seriously.
"We are working together in partnership with our colleagues in the Organised Crime Task Force (OCTF) to stop the damage to our local businesses and environment."
He warned motorists who buy laundered fuel to be aware of the damage illegal fuel can cause to vehicles.
"That is why I would urge members of the public to contact our free telephone hotline 0800 59 5000 and let us know of any fuel misuse or suspicious activities in their area."
Officers from the Environmental Heritage Agency attended on site.
Enquiries into the illegal operation are continuing.