  The Interstellar Medium (ISM) comprises gases at different temperatures and
densities, including ionized, atomic, molecular species, and dust particles.
The neutral ISM is dominated by neutral hydrogen and has ionization fractions
up to 8%. The concentration of chemical elements heavier than helium
(metallicity) spans orders of magnitudes in Galactic stars, because they formed
at different times. Instead, the gas in the Solar vicinity is assumed to be
well mixed and have Solar metallicity in traditional chemical evolution models.
The ISM chemical abundances can be accurately measured with UV absorption-line
spectroscopy. However, the effects of dust depletion, which removes part of the
metals from the observable gaseous phase and incorporates it into solid grains,
have prevented, until recently, a deeper investigation of the ISM metallicity.
Here we report the dust-corrected metallicity of the neutral ISM measured
towards 25 stars in our Galaxy. We find large variations in metallicity over a
factor of 10 (with an average 55 +/- 7% Solar and standard deviation 0.28 dex)
and including many regions of low metallicity, down to ~17% Solar and possibly
below. Pristine gas falling onto the disk in the form of high-velocity clouds
can cause the observed chemical inhomogeneities on scales of tens of pc. Our
results suggest that this low-metallicity accreting gas does not efficiently
mix into the ISM, which may help us understand metallicity deviations in nearby
coeval stars.
