IRA fuel smugglers are back in business almost two years after Irish Revenue Commissioners thought they had stopped the trade with a new "silver bullet" isotope dye.
Five tons of toxic sludge from "washing" dye from agricultural diesel were dumped by a road near the Creggan River, just south of the border in Co Louth recently.
The waste, which contains carcinogenic petrochemical compounds, was left on the roadside in 1,000-litre containers at Balregan, north west of Dundalk.
In the past, Customs estimated each container represented 100,000 litres of processed diesel, so this month's find represented the waste from an operation which could have washed 500,000 litres of diesel.
Industry sources said the "diesel" being sold by the Provo smugglers is mixed with cheap kerosene (paraffin) which can be bought wholesale for as little as 35c (30p) a litre. While the isotope marker introduced in April 2015 cannot be removed, sources said hauliers under severe financial pressure are prepared to take the risk of being caught because of the economic advantage cheaper illicit fuel gives them.
South Armagh gangs are also, again, opening outlets around the Republic and Northern Ireland as they see opportunities for illicit fuel as price differentials loom with the approach of Brexit.
At the height of the fuel laundering about a decade ago, it was estimated the UK exchequer was losing up to £350m a year and half the filling stations in Northern Ireland were selling their washed product.
Corresponding figures were never released by Revenue in the Republic, but at one stage there were more than 130 filling stations in the Republic owned or controlled by south Armagh smugglers.