Ten tons of toxic tar was discovered on Wednesday after being dumped on a side road in Koropi, east of Athens.
The containers were spotted by an Attiki Odos employee who notified authorities.
They believe that the substance is what was left over from fuel smugglers’ efforts to convert bunker fuel into heating oil.
Officials estimate that the smugglers were able to produce at least 100 tons of illegal heating fuel.
The toxic liquid had begun to spill onto the road and into a nearby field.
Specially equipped inspectors, wearing body suits, arrived at the scene to collect samples.
Authorities said that the tar would be mopped up by Thursday.
Officials said that until now this type of illegal activity had taken place in western Attica, near the oil refineries.
This is the first time they have encountered this form of fuel smuggling in eastern Attica.