Related: in Sweden they filled a cave (an old mine?) with water and are using that as heat storage: https://www.youtube.com/watch?v=VSdKL0Nnk-k https://www.yout

Related: in Sweden they filled a cave (an old mine?) with water and are using that as heat storage: https://www.youtube.com/watch?v=VSdKL0Nnk-k https://www.youtube.com/watch?v=VSdKL0Nnk-k