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An assembly for storing thermal energy is provided. The assembly includes a spatial heat reservoir, a solid natural material, which is designed to store heat, and a fluid, which is designed to transfer thermal energy to the heat storage material and to remove thermal energy from the heat storage material. The heat storage material is arranged within the heat reservoir. The heat reservoir has at least one inlet and at least one outlet, through which the fluid is conducted in order to store energy and remove energy. In order to store energy, heated fluid is introduced into the heat reservoir through the inlet, thermal energy is transferred to the heat storage material there, and the fluid is conducted out of the heat reservoir through the outlet. Correspondingly, in order to remove enemy, cool fluid is introduced into the heat reservoir through the inlet, thermal energy is absorbed from the heat storage material there, and the fluid is conducted out of the heat reservoir through the outlet. An igneous rock is used as the heat storage material.