Under a hill in Lucens, in the Gros-de-Vaud area, a cave carved into molasse rock once housed the dream of a fully Swiss nuclear reactor. This dream ended abruptly on January 21, 1969, during an accident that is now estimated to have been between levels 4 and 5 on the international nuclear event scale. Since then, the same cave has become a repository for the archives, artworks, and archaeological collections of the canton of Vaud. The site, once home to the Lucens experimental nuclear power plant (CNEL), was connected to the Swiss electrical grid on January 29, 1968, driven by the ambition of creating a national atomic industry without relying on American licenses or imported technology. The reactor, a heavy water and natural uranium design cooled with carbon dioxide, was housed in an old military gallery. On January 21, 1969, the Lucens reactor experienced a partial melting of its fuel element number 59 due to water infiltration in the primary circuit. The accident, which would later be classified between levels 4 and 5 on the International Nuclear Event Scale (INES), did not cause any casualties because the cave was empty at the time of the explosion. The day was meant to mark the final start-up of the installation after a maintenance stop, but when the reactor's power had reached nearly 40%, an accident occurred, causing several explosions. At 5:14 p.m., the power reached 12 MW. Six minutes later, an automatic shutdown was triggered. The cause traced back to a maintenance operation in late October 1968, during which water had infiltrated the primary circuit and gradually eroded the magnesium sheaths around the metallic uranium, eventually blocking the cooling channels of element number 59. The cave was empty at the time of the explosion, which limited the human impact of the accident. A highly radioactive gas mixture spread throughout the cave, but no one was present. Measurements taken afterward showed only a slight increase in radioactivity, well below the limit values, and the effects on personnel and the environment were considered negligible. The final report of the investigation commission was published only in 1979, a decade after the event. In the meantime, the authorities had classified the event as a mastered technical incident, without giving the public the full measure of what had occurred under the Vaud hill. The INES scale, which now allows for the classification of the severity of nuclear accidents, did not exist in 1969: it was developed in 1990 by the International Atomic Energy Agency and the Nuclear Energy Agency of the OECD. Retrospectively, evaluations place Lucens between levels 4 and 5 on a seven-level scale, making it one of the most serious civil nuclear accidents ever recorded in the world. The dismantling of the site took place over several years after the accident. The 72 intact fuel elements were sent to the Eurochemic reprocessing center in Mol, Belgium. Approximately 230 drums of low-level radioactive waste were packaged, along with six containers containing, among other things, 60 kilograms of uranium from the damaged element 59. Two of the three caves were then filled with concrete between 1991 and 1993. The site was officially cleared in 1995, except for a parcel, and the last radioactive waste left Lucens in 2003 to join the intermediate storage facility in Würenlingen, in the canton of Aargau. The third cave, which once housed the turbine, had an unexpected fate. The canton of Vaud installed in the turbine cave a repository for its museums, libraries, and archives. Today, the site serves as a reserve for nearly all the cantonal cultural institutions, from the Museum of Archaeology and History to the Canton and University Library. An old ticket machine, the first safe of the Vaudois Cantonal Bank, and Japanese swords now stand alongside archaeological remnants in the same underground galleries. The Swiss Cinémathèque even temporarily stored films there, taking advantage of the stable coolness and total darkness of the place. Under the hill of Lucens, the fission of the atom gave way to the preservation of heritage, a reversal that few industrial sites can claim with such clarity.