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Mountain Lake, Virginia is a small, unique, oligotrophic, subalpine ecosystem in the southern Appalachians. Previous studies have disclosed that this lake has manifested periodic prolonged low water levels during the several thousand years of its existence. The most recent low water level occurred during the drought years of 1999-2002. Measurements of lake level, precipitation, and other meteorological data including calculated evapotranspiration in the lake basin from 2/19/02 to 8/31/03 have enabled estimation of net subterranean water losses presumably through cracks between Clinch sandstone boulders and/or the recently discovered deep hole at the northwest end of Mountain Lake. These net losses reflect the balance between total losses and any gains from springs and boulder cracks not quantified in this study. Scuba divers have documented the existence of these cracks and the deep hole. Subterranean net water losses of about 0.04-0.05 m3/s (634-792 gpm) apparently occur year-round.