Members of Sweden's Coast Guard found a 200km long and 10km wide, unknown substance floating in the middle of the Gulf of Bothnia, according to Swedish public broadcaster SR.
Characterising the presumably spilled substance "enormous," SR reported (siirryt toiseen palveluun) that, apart from spills of mineral-based oil, the incident was likely the largest of its kind, according to the Swedish Coast Guard.
Kalle Ahola, West Finland Coast Guard District's field director, said on Thursday that the substance was spotted in satellite images taken about a day-and-a-half ago.
"The Swedish Coast Guard has been monitoring the situation from the air. There's not certainty or even suspicions about what the substance could be, but it's not likely mineral-based oil, so there is no need to control measures," Ahola explained.
Most of the substance had evaporated by Thursday, but samples were taken and are currently being analysed, according to Ahola.
Based on the initial satellite images, the trail of the unknown substance, floating in the middle of the Gulf, was located between the Finnish port cities of Pori and Rauma.
Ahola said he did not know from where the substance may have come.