  We present a novel source of dark energy, which is motivated by the
prevalence of hidden sectors in string theory models and is consistent with all
of the proposed swampland conjectures. Thermal effects hold a light hidden
sector scalar at a point in field space that is not a minimum of its zero
temperature potential. This leads to an effective "cosmological constant", with
an equation of state $w=-1$, despite the scalar's zero temperature potential
having only a 4D Minkowski or AdS vacuum. For scalar masses $\lesssim \mu$eV,
which could be technically natural via sequestering, there are large regions of
phenomenologically viable parameter space such that the induced vacuum energy
matches the measured dark energy density. Additionally, in many models a
standard cosmological history automatically leads to the scalar having the
required initial conditions. We study the possible observational signals of
such a model, including at fifth force experiments and through $\Delta N_{\rm
eff}$ measurements. Similar dynamics that are active at earlier times could
resolve the tension between different measurements of $H_0$ and can lead to a
detectable stochastic gravitational wave background.
