Extraction involves methods such as underground or surface mining. Extraction of mineral resources reflects the quantity ofthe resource physically removed from the deposit during a period of time (usually one year). The difference between the opening and closing stocks of mineral resources for a particular year result largely from extraction. However, new discoveries, reappraisals and reclassifications of stocks, as well as catastrophic losses, can also influence the difference between opening and closing stocks. Main sources of statistics about stocks of mineral resources are geological surveys and inventories, as well as economic statistics on mining and quarrying.