The structure in the electron-positron vacuum for overcritical external fields is shortly reviewed. Emphasis is given to the structure of the baryon vacuum in the vicinity of a nucleus, particularly if the nucleus is compressed: thousands of bound states of negative energy emerging out of the negative energy continuum represent the highly correlated vacuum in this case. The implication of this correlation structure for ordinary and superheavy nuclei, for the production of antimatter (i.e. antinuclei) and for highly strange hypernuclei is discussed. An extension of the Periodic System into these new areas is anticipated.

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