Geochemical and acoustic evidence for the occurrence of methane in sediments of the Polish sector of the southern Baltic Sea**The research was supported by the Polish Ministry of Science and Higher Education grant No. N306 441638 entitled ‘Occurrence and transformations of methane in sediments of the southern Baltic Sea’, by the BONUS project ‘BALTIC GAS. Methane emission in the Baltic Sea: Gas storage and effects of climate change and eutrophication (2009–2011)’ and by the National Science Centre grant No. DEC-2011/03/N/ST10/05633 ‘Acoustic recognition of gas-bearing sediment forms in the southern Baltic Sea and quantitative assessment of gas outflows’.