A few decades ago, Antarctic ice sheets were expected to grow as the atmosphere warmed and increasing poleward moisture transport added snowfall to regions that would remain below freezing year-round. Concerns about their sensitivity to climate change were centered on air temperature and on glacially paced ice dynamics. Southern Ocean roles were relegated to iceberg transport, a mix of melting and freezing under ice shelves buffered by the frigid shelf waters generated by sea ice production, and slow sea level rise by other forcing. At that time, observations were lacking in the remote Amundsen Sea, where difficult ice conditions have vexed explorers for more than 200 years. Mapping of its ocean structure and circulation began in 1994, revealing that “warm” Circumpolar Deep Water has access to its continental shelf. Glacially scoured troughs in the seafloor provide conduits for that seawater to melt regional ice shelves far more rapidly near their deep grounding lines. Coincident satellite data showed the ice shelves were thinning, in turn leading to accelerated glacier flow and loss of grounded ice to the sea. Repeated measurements and modeling suggest ocean changes that could impact the stability of the marine-based West Antarctic Ice Sheet.