  The term "unstable neutral media" (UNM) has traditionally been used to
describe the transient phase formed between the warm and cold neutral hydrogen
(HI) phases and has not been the focus of HI studies. However, recent
observations suggest that the UNM phase not only has a significantly
longer-than-expected lifetime but also occupies at least 20\% to 40\% of both
the volume and mass fraction of HI. In this paper, we argue that the existence
and dominance of the UNM can be explained by the presence of strong turbulence
using an energy balance argument. The mass fraction of UNM is directly
proportional to the turbulent velocity dispersion $\sigma_v$: mass fraction of
UNM $\propto \sigma_v^{\frac{2n}{1+n}}$, where $n$ is the absolute value of the
adiabatic index in the unstable phase. We discuss the implications of
long-lived unstable thermal phases on ISM physics, including cold dense
filament formation, cosmic ray acceleration, and measurement of galactic
foreground statistics.
