Many physics text books reference to the concept of the Dirac sea as explanation of negative frequency solutions of the Dirac equation. It is supposed to be a bottomless "sea" of filled electron states. The other physical implications of this theory of the vacuum are never explored in a more than handwaving detail. Clearly the concept implies infinite electron density. The associated infinite negative charge density must be compensated by an equally infinite positive charge density. Effects of electron correlation would likely alter the properties of the hole, giving an effective mass different from that of an electron. What are the n&k values of the Dirac sea? We observe that the vacuum is perfectly transparent. How can this be consistent with the omnipresence of an infinite electron density ? In view of this blatant lack of physical motivation, why is this idea still surfacing in physics at all?
See for example Greiner, Relativistic quantum mechanics. Wave Equations, page 112 and others.