PRINCETON, N.J. — A research team led by Princeton University and the University of Arizona has mapped the massive amount of water hidden beneath the lower 48 states. According to a study released yesterday, Jan. 26, 2026, the mainland United States contains approximately 73,500 cubic miles (306,500 cubic kilometers) of groundwater.

This total volume of underground water is equivalent to 13 times the capacity of all the Great Lakes combined. The estimate is also nearly seven times the total amount of water that flows every year from every river on Earth.

Lead researchers Reed Maxwell and Laura Condon used a highly detailed map to calculate groundwater levels down to a depth of about 1,286 feet (392 meters). The map has a "30-meter resolution," meaning it can show details as small as a baseball diamond. To reach these conclusions, the team trained an advanced computer model using roughly one million data measurements collected between 1895 and 2023. This data combined historical well records, satellite imagery, and climate maps.

Tracking Human Impact

The new model also tracks how human activities affect water supplies. By analyzing historical data, the system recognized patterns of human water usage, including how much water people pump out and how quickly they are using it up.

To show why this detail matters, the team tested a less precise map. They found that standard models with less detail missed about 18% of the water found in their high-resolution version.

Shallow Water Reserves

The study further revealed that about 40% of the mainland United States sits over a "water table" less than 33 feet (10 meters) deep. The water table is the level where the ground becomes completely soaked with water. Identifying these shallow areas is important because they show where underground water is close enough to reach plant roots or the ground's surface.

Researchers said the new data provides a better starting point for local officials to manage water supplies and plan for watering crops. The study and the new groundwater map can be found online at https://www.nature.com/articles/s43247-025-03094-3.