Researchers from Princeton University and the University of Arizona have created a simulation that maps underground water on a continental scale. The result of three years' work studying groundwater from coast to coast, the findings plot the unseen path that each raindrop or melted snowflake takes before reemerging in freshwater streams, following water from the land surface to depths far below and back up again, emerging up to 100 miles away, after spending from 10 to 100,000 years underground.