Solid mercury in the rhombohedral structure is unbound within the self-consistent field (Hartree-Fock) approximation. The metallic binding is entirely due to electronic correlations. We determine the cohesive energy of solid mercury within an ab initio many-body expansion for the correlation part. Electronic correlations in the 5d shell contribute about half to the cohesive energy. Relativistic effects are found to be very important. The obtained value agrees very well with the experimental cohesive energy. (C) 2004 Elsevier B.V. All rights reserved.