After the German re-unification in 1990, the Reichstag building in Berlin was completely refurbished to house again the German Parliament, the “Bundestag”. The design of this work was in the hands of the British architect Sir Norman Foster, and since the first presentation of his plans in 1992 the energy concept included a geothermal component, i.e. the storage of thermal energy in the underground. Two aquifers at different depth are used to store cold (ca. 60 m) and heat (ca. 300 m). The paper explains the system concept and the realised installation and presents first results of a monitoring campaign. The underground storage is operational since 1999, however, the full capacity of the total system and the final operational strategy could not be tested before completion of the energy network and all buildings involved in 2003. Both storage systems, after minor teething problems, performed to satisfaction. The monitoring was of great importance to detect and solve some operational inaccuracies and to optimise the system hardware as well as the operational strategies.