However, several
studies suggest that significant amounts of CH4, produced within the Earth's crust (mainly by bacterial and thermogenic processes), are released into the atmosphere through faults and fractured rocks, mud
volcanoes on land and the seafloor,
submarine gas seepage, microseepage over dry lands and geothermal seeps (Etiope and Klusman, 2002; Etiope, 2004; Kvenvolden and Rogers, 2005).