This project represents our proposition for an international competition launched by the municipality of Stockholm, for the extension of the library, built by Gunnar Asplund in 1928. The designated building was to be buried in the Obervatoirekullen, hill of the Observatory situated behind Odengatan. Only visible and present in the landscape: the entrance of the new building. Its curved, wave-like glass shape is the continuation of the natural hill, connecting the building of Asplund and the four buildings of Lallerstedt that we decided to preserve, because it is a part of the city heritage. It opens on a wide square on Odengatan. The internal circulation of the buried building follows the curves of the hill and is enlightened by slots in the hill. Only the cafeteria, at the last level, emerges out of the ground as a detached house at the top of the hill.