Chemical, microbiological, and geophysical measurements have been carried out on sediment cores collected from Holyhead Harbour and the Western Irish Sea, where acoustic subbottom profiling has established the presence of large areas of acoustically turbid sediments, commonly referred to as “gassy” sediments. Gas analysis of these cores have shown that the acoustic turbidity was most probably due to high concentrations (>100 nM/mL) of methane occurring at subsurface depths.