In 1994, ocean measurements near Antarctica's Pine Island Glacier showed that the ice shelf buttressing the glacier was melting rapidly1. This melting was attributed to the presence of relatively warm, deep water on the Amundsen Sea continental shelf. Heat, salt and ice budgets along with ocean modelling provided steady-state calving and melting rates2,3. Subsequent satellite observations and modelling have indicated large system imbalances, including ice-shelf thinning and more intense melting, glacier acceleration and drainage basin drawdown4-10. Here we combine our earlier data with measurements taken in 2009 to show that the temperature and volume of deep water in Pine Island Bay have increased. Ocean transport and tracer calculations near the ice shelf reveal a rise in meltwater production by about 50% since 1994. The faster melting seems to result mainly from stronger sub-ice-shelf circulation, as thinning ice has increased the gap above an underlying submarine bank on which the glacier was formerly grounded 11. We conclude that the basal melting has exceeded the increase in ice inflow, leading to the formation and enlargement of an inner cavity under the ice shelf within which sea water nearly 4 °C above freezing can now more readily access the grounding zone.