The atomic-scale process of vacancy clustering, of fundamental importance in conditions of quenching, irradiation, and plastic deformation, is studied in aluminum. Clustering is known to lead to large crystalline defects such as dislocation loops, but the early stages of the process are still largely unknown. Using a combination of molecular dynamics simulations, the activation-relaxation technique and kinetic Monte Carlo simulations, a specific cluster, containing five vacancies and forming a local body-centered-cubic cell in the face-centered-cubic lattice, is found to play a crucial role. It is revealed that this cluster of very high stability is the nucleus for the growth of larger clusters during annealing of quenched supersaturated samples.