In Utrecht knowledge institutions, companies, the municipality and the province of Utrecht join forces to improve the performance of thermal energy storage in a worldwide unique way. On three separate locations, a three-dimensional underground fiber network is installed that will measure the temperature in the subsoil throughout the seasons. In doing so for the first time it becomes clear how the energy balance in the subsoil evolves around ATES systems throughout the seasons and how the geothermal energy potential per area can be optimally utilized. A breakthrough, aimed at increasing the energy and financial efficiency of thermal energy storage systems. The Utrecht Sustainability Institute has initiated this project in order to monitor the subsoil at Kantorenpark Rijnsweerd, Utrecht Science Park and in the station area of Utrecht CS.