The Laurentian Great Lakes hold over 20% of the world’s unfrozen surface freshwater and are crucial for climate regulation and ecosystem health. While many models have been developed to study water movement in this region, they usually focus on surface processes like rainfall, runoff, and evaporation. Groundwater, though essential for flowing rivers, sustaining lake levels, and supplying nearshore environments, is often left out, especially in large-scale studies. This project introduces a groundwater model that covers the entire Great Lakes Basin. It combines U.S. and Canadian datasets on geology, water levels, and properties below the surface to represent the groundwater system better. The model shows how groundwater elevations are closely tied to regional geology and reveals how groundwater flows across the basin, a relationship commonly overlooked in large-scale models. By focusing only on groundwater at the basin scale, this model helps fill a major gap in our understanding. It offers a valuable tool for future research, including studies that connect groundwater with surface water and climate models. Most importantly, it supports better planning and management of freshwater resources in one of the world’s most important freshwater systems.