A massive spill of an unknown substance has been detected in the Baltic Sea off the coast of Sweden, the country’s coastguard said on Thursday. Covering a surface area of 30 sq miles (77 sq km) in both Swedish and Finnish waters, the spill was detected on Wednesday in the Bothnian Sea. “What the spill consists of is still not clear but it is not mineral oil, and there is currently no immediate threat of landfall,” the coastguard said in a statement. It said it had mapped the spill using planes and collected samples, adding that it would not be able to comment on which measures to deploy until after the samples had been analysed. A preliminary investigation into environmental crimes has also been launched. “Among other things, it is being investigated which ships have been in the area and what cargo they have had,” the coastguard said. Later on Thursday, it said the spill was no longer visible and that spills other than oil were increasing. “New types of fuel being transported at sea are increasing - for instance biofuel - and when in contact with water, they exhibit a great variety of behaviours which makes it more complicated to quickly establish what substance it is,” Jonatan Tholin, head of the coast guard investigation, said in a statement.