Earth’s glaciers are currently retreating to positions not yet observed by humans and thereby contributing considerably to global sea-level rise. Recent results of paleo-studies in the Amundsen Sea, West Antarctica, undertaken mainly by the International Thwaites Glacier Collaboration (ITGC), demonstrate a possible range of ice-sheet dynamical behavior not captured by the observational record. Amongst several new insights, bathymetric mapping of the paleo-ice sheet bed revealed larger pathways for incursions of warm ocean water than previously estimated. High-resolution mapping of the bed revealed faster past retreat of Thwaites Glacier than observed in recent decades. In addition, marine sediment cores documented that Thwaites Glacier’s latest retreat began in the 1940s. These results highlight the value of the paleo-record for understanding modern ice sheets and their sensitivity to climatic forcing.