A survey of the southern Amundsen and Bellingshausen seas in early 1994 included the first deepoceanographic measurements along the calving front of Pine Island Glacier, which drains~2x10^5 km^2 of the marine-based West Antarctic Ice Sheet. The measurements reveal a cycloniccirculation in Pine Island Bay and >1 degC Circumpolar Deep Water flowing beneath and rapidlymelting the floating base of this deep-rooted glacier. Dissolved oxygen measurements show asubsurface oxygenation of the water column resulting from air bubbles in the melting ice. Seawateroxygen isotope analyses imply average delta18O values of -29 permil for precipitation on thecatchment basin that subsequently melts at the glacier base. Application of a two-dimensionalthermohaline circulation model to the sub-ice shelf cavity supports average basal melt rates inexcess of 10 m/a calculated by two other methods. This melt rate is more than 5 times the averagereported for any Antarctic ice shelf, drives Circumploar Deep Water upwelling on the continentalshelf, and generates melt-laden outflows that are well above surface freezing temperature.