  We describe a new family of thin-wall instantons, with negative tension
bubble walls, that mediate tunneling between Minkowski or de Sitter vacua. Some
of these instantons can have exponentially enhanced tunneling rates, and would
dominate the Euclidean path integral, representing a severe non-perturbative
instability in any theory which supports them. We give two constructions of
these instantons in theories which are perturbatively stable, but violate the
null energy condition. One construction uses a scalar field theory with
higher-derivative kinetic term, and is similar to the Coleman-de Luccia
positive tension instanton. The other construction employs a negative-tension
boundary arising from Z_2 orbifolding: it resembles a "bubble of nothing" which
nucleates and grows, consuming the surrounding de Sitter or Minkowski
spacetime. We explain how the spectral flow for fluctutations about the
thin-wall tunneling solutions automatically protects causality, for both
positive and negative tension instantons. We comment briefly on the relation of
these solutions to a Kalzua-Klein "bubble of nothing," cosmological models
relying on ghost condensates, and string theory orientifolds.
