To understand Antarctica’s present and future contribution to sea level rise, it is crucial to understand its fastest-moving glaciers, known as ice streams. The sliding of ice streams is controlled by a layer of water at the base of the ice, contained in subglacial channels or lakes, or within porous sediments. In addition, thick layers of porous rock known as sedimentary basins lie beneath these glaciers, containing large volumes of groundwater. This groundwater includes both freshwater, which is exchanged with the layer at the base of the ice, and seawater. In this talk, I will discuss how mathematical modelling can provide insight into modelling these groundwater systems, with implications for Antarctic ice loss and sea level rise. In particular, I will focus on modelling groundwater flow in sedimentary basins beneath glaciers, showing how a mathematical model can predict the trapping of seawater in these basins and the exchange of freshwater with the system at the base of the ice. These predictions can be compared with field data to learn more about the properties of these sedimentary basins and their influence on ice streams.