  We discuss materials which owe their stability to external fields. These
include: 1) external electric or magnetic fields, and 2) quantum vacuum
fluctuations in these fields induced by suitable boundary conditions (the
Casimir effect). Instances of the first case include the floating water bridge
and ferrofluids in magnetic fields. An example of the second case is taken from
biology where the Casimir effect provides an explanation of the formation of
stacked aggregations or "rouleaux" by negatively charged red blood cells. We
show how the interplay between electrical and Casimir forces can be used to
drive self-assembly of nano-structured materials, and could be generalized both
as a probe of Casimir forces and as a means of manufacturing nanoscale
structures. Interestingly, all the cases discussed involve the generation of
the somewhat exotic negative pressures. We note that very little is known about
the phase diagrams of most materials in the presence of external fields other
than those represented by the macroscopic scalar quantities of pressure and
temperature. Many new and unusual states of matter may yet be undiscovered.
