We explore the formation of superbubbles through energy deposition by multiple supernovae (SNe) in a uniform medium. We use total energy conserving, 3-D hydrodynamic simulations to study how SNe correlated in space and time create superbubbles. While isolated SNe fizzle out completely by ∼ 1 Myr due to radiative losses, for a realistic cluster size it is likely that subsequent SNe go off within the hot/dilute bubble and sustain the shock till the cluster lifetime. For realistic cluster sizes, we find that the bubble remains overpressured only if, for a given n g0 , N OB is sufficiently large. While most of the input energy is still lost radiatively, superbubbles can retain up to ∼ 5 − 10% of the input energy in form of kinetic+thermal energy till 10 Myr for ISM density n g0 ≈ 1 cm −3 . We find that the mechanical efficiency decreases for higher densities (η mech ∝ n −2/3 g0 ). We compare the radii and velocities of simulated supershells with observations and the classical adiabatic model. Our simulations show that the superbubbles retain only 10% of the injected energy, thereby explaining the observed smaller size and slower expansion of supershells. We also confirm that a sufficiently large ( 10 4 ) number of SNe is required to go off in order to create a steady wind with a stable termination shock within the superbubble. We show that the mechanical efficiency increases with increasing resolution, and that explicit diffusion is required to obtain converged results.