The ambition to achieve net-zero emissions has accelerated the transition toward renewable energy production, but the intermittency of most renewable energy technologies, coupled with a lack of storage technology, often results in curtailment during times of high production. Long-duration energy storage is crucial for balancing the grid during reduced renewable energy generation periods. Repurposing abandoned flooded mine workings for low-carbon energy applications has gained significant attention, especially in countries with strong mining legacies. However, most projects focus on geothermal heat recovery rather than energy storage and are mostly in the pilot phase or characterized by modest thermal throughput, typically under several tens of kW. The STEaM (GigaWatt-Hour Subsurface Thermal Energy storAge: Engineered structures and legacy Mine shafts) project pioneers the use of mine shaft thermal energy storage (MSTES) to help balance the electrical grid and decarbonize homes and businesses. STEaM aims to perform thermal–hydraulic-chemical–mechanical (THCM) modeling, and take an energy-systems approach to determine the optimal integration of this resource into local energy grid and heat network systems. This study highlights previous examples of mine shaft heat extraction in the UK and demonstrates the potential for MSTES to contribute to the low-carbon energy transition.