  We investigate the formation via tunneling of inflating (false-vacuum)
bubbles in a true-vacuum background, and the reverse process. Using effective
potentials from the junction condition formalism, all true- and false-vacuum
bubble solutions with positive interior and exterior cosmological constant, and
arbitrary mass are catalogued. We find that tunneling through the same
effective potential appears to describe two distinct processes: one in which
the initial and final states are separated by a wormhole (the Farhi-Guth-Guven
mechanism), and one in which they are either in the same hubble volume or
separated by a cosmological horizon. In the zero-mass limit, the first process
corresponds to the creation of an inhomogenous universe from nothing, while the
second mechanism is equivalent to the nucleation of true- or false-vacuum
Coleman-De Luccia bubbles. We compute the probabilities of both mechanisms in
the WKB approximation using semi-classical Hamiltonian methods, and find that
-- assuming both process are allowed -- neither mechanism dominates in all
regimes.
