The discovery and large-scale synthesis of fullerenes have aroused interdisciplinary interest in these closed-cage molecules(1-6). C-60 can be photopolymerized into a form in which the cages are thought to be linked by cyclic C-4 units in a [2 + 2] cycloaddition(7), provoking theoretical studies of the C-60 dimer(8-15), the smallest subunit of such a polymer. The C-60 dimers C120O (refs 16, 17), C121H2 (ref. 17) and C120O2 (ref. 18) have been reported, in which the two C-60 molecules are linked by, respectively, a furan group, a cyclopentane ring and a cyclobutane ring plus two oxygen bridges; but the simplest dimer, C-120 linked by a cyclobutane ring alone, has not so far been observed, We now report that this dumb-bell-shaped molecule can be synthesized by a solid-state mechanochemical reaction of C-60 with potassium cyanide. Our X-ray structural analysis shows that the C-4 ring connecting the cages is square rather than rectangular-the latter is predicted theoretically(8,9,13-15). The dimer dissociates cleanly into two C-60 molecules on heating or one-electron reduction, but in the gas phase during mass-spectrometric measurements it undergoes successive loss of C-2 units, shrinking to even-numbered fullerenes such as C-118 and C-116 in a sequence similar to that seen for other large fullerenes(19,20).