  The positioning of one or more bubbles inside a many fermion or boson system,
does not affect the volume, surface or curvature terms in the liquid drop
expansion of the total energy. If Coulomb effects are irrelevant, the only
contribution to the ground state energy of a system with one or more voids
arises from quantum effects, which is similar to the Casimir effect in vacuum
or in critical phenomena. We discuss the characteristics of such systems, the
interplay among various effects, such as shell corrections and chaotic
behavior, and briefly mention the role of the temperature and pairing.
