  We explore the fate of the universe given the possibility that the density
associated with `dark energy' may decay slowly with time. Decaying dark energy
is modeled by a homogeneous scalar field which couples minimally to gravity and
whose potential has {\em at least one} local quadratic maximum. Dark energy
decays as the scalar field rolls down its potential, consequently the current
acceleration epoch is a transient. We examine two models of decaying dark
energy. In the first, the dark energy potential is modeled by an analytical
form which is generic close to the potential maximum. The second potential is
the cosine, which can become negative as the field evolves, ensuring that a
spatially flat universe collapses in the future. We examine the feasibility of
both models using observations of high redshift type Ia supernovae. A maximum
likelihood analysis is used to find allowed regions in the $\lbrace m,
\phi_0\rbrace$ plane ($m$ is the tachyon mass modulus and $\phi_0$ the initial
scalar field value; $m\sim H_0$ and $\phi_0\sim M_P$ by order of magnitude).
For the first model, the time for the potential to drop to half its maximum
value is larger than $\sim 8$ Gyrs. In the case of the cosine potential, the
time left until the universe collapses is always greater than $\sim 18$ Gyrs
(both estimates are presented for $\om = 0.3$, $m/H_0 \sim 1$, $H_0 \simeq 70$
km/sec/Mpc, and at the 95.4% confidence level).
