A methane content of a bottom sample comprising methane hydrate crystals is determined by:—taking a core sample (5) from a bottom sediment (3) in a deepwater area;—storing the core sample (5) in a storage chamber (4);—lifting the storage chamber (4) to a predetermined waterdepth (BGHZ=Base of Gas Hydrate stability Zone) at which any methane hydrate crystals in the core sample (5) dissociate into water and methane; and—measuring an amount of methane released by the lifted core sample (5).