The greatest energy source by far is matter itself. This energy, however, can be released only by the processes of nuclear fission (~0.1%), nuclear fusion (~1%), or the annihilation of some or all of the matter in the volume V by matter- antimatter collisions (100%). According to the mass-energy equivalence, this energy is described by E = mc, where c is the speed of light. In terms of density, m = ρV, where ρ is the mass per unit volume, V is the volume of the mass itself.