The global energy transition, driven by the temporal decoupling of production and demand from wind and solar power, creates a growing need for storage capacity and intensifies challenges for power grids. To complement short-term battery storage, the HEATbucket project is researching an efficient and complementary solution for urban areas: the storage of energy in the form of heat. This heat is charged either via renewable electricity using Power-to-Heat or through renewable heat sources such as solar thermal energy. To make underground heat storage practical in densely built-up urban areas, HEATbucket is developing pragmatic constructional, thermodynamic, and hydrogeological solutions. The goal is to decentralize the heat supply and improve the CO2\u200b balance, with intensive investigation into feasibility and environmental impacts on the subsurface and groundwater.