This paper reports on the first large-scale project in the U.S. for utilizing water from abandoned mines for geothermal applications. This project proved the high potential of turning water in deep abandoned mines into a renewable energy resource, which is safer, greener, and more abundant than other conventional low-enthalpy geothermal applications. In this paper, a real demonstration project is introduced for recovering geothermal energy from the mine water for heating and cooling to a 1022 m2 building in the Upper Peninsula of Michigan. A field test in a mine shaft with a depth of 1219.2 m is then presented to show the key issue in the use of the mine water as a geothermal resource: the temperature distribution. Complex multiphysics simulation with unique non-isothermal hydrodynamics is conducted to provide a physical explanation for the data obtained in the field test. Simulation results shed light on the scientific myth regarding water stratification and energy flow observed in the field study.