The Boule is a large spherical steel building, 25 meters in diameter, an icon for CEMES. The construction of the Boule was initiated by Gaston Dupouy, and the building was inaugurated in 1959 by General de Gaulle. It was designed to house the 1-million-volt electron microscope that operated from 1960 to 1991. The original microscope was later dismantled, but the electron accelerator has been preserved and still stands under the vault of the sphere.