  Wormholes -- solutions to the euclidean Einstein equations with non-trivial
topology -- are usually assumed to make real contributions to amplitudes in
quantum gravity. However, we find a negative mode among fluctuations about the
Giddings-Strominger wormhole solution. Hence, the wormhole contribution to the
euclidean functional integral is argued to be purely imaginary rather than
real, which suggests the interpretation of the wormhole as describing the
instability of a large universe against the emission of baby universes.
