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Previous estimates of the world-wide input of oil to the marine environment by natural seeps ranged from 0 ·2 to 6 ·0 million (metric) tonnes per year with a 'best estimate' of 0 ·6 million tonnes per year. Based on considerations of the availability of oil for seepage from the world's known and assumed oil resources, we believe that the world-wide natural oil seepage over geological time should be revised to about 0 ·2 million tonnes per ),ear with a range upward or downward of a factor of ten leading to estimates between 0 ·02 and 2 million tonnes per year. Our estimate of the amount of oil eroding from the land and being transported to the oceans is about 0 ·05 million tonnes per year with an order of magnitude uncertainty. Therefore, while the uncertainties are large, we estimate that the total amount of oil entering the marine environment by natural, geological processes, is about 0 ·25 million tonnes per year, and the estimate may range from about 0 ·025 to 2 ·5 million tonnes per year.