In 1985, Unocal drilled the first deep test well in the western moat of Long Valley caldera, ∼1 km east of the Inyo Craters. Well IDFU 44-16 penetrated a thinner section of Bishop Tuff than that found in earlier deep exploration wells on the resurgent dome. Precaldera volcanics were encountered at 1183 m and metamorphic basement at 1615 m. The precaldera volcanics yielded a radiometric age of 1.98±0.1 Ma and are tentatively correlated with Tertiary andesites and dacites on the caldera's western wall. These stratigraphic relations suggest that the caldera's structural margin is at least 4 km east of the current topographic margin. Temperatures reach a maximum of 218°C at 1100 m in IDFU 44-16; however, temperature reversals occur below this depth. Temperature and permeability measurements indicate a lateral outflow of hot water over cold recharge near the caldera's structural margin. While these discoveries support the existence of hightemperature fluids in the western moat of Long Valley caldera, the ultimate source of the geothermal system remains to be discovered.