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The Interstellar Medium (ISM) comprises gases at different temperatures anddensities, including ionized, atomic, molecular species, and dust particles.The neutral ISM is dominated by neutral hydrogen and has ionization fractionsup to 8%. The concentration of chemical elements heavier than helium(metallicity) spans orders of magnitudes in Galactic stars, because they formedat different times. Instead, the gas in the Solar vicinity is assumed to bewell mixed and have Solar metallicity in traditional chemical evolution models.The ISM chemical abundances can be accurately measured with UV absorption-linespectroscopy. However, the effects of dust depletion, which removes part of themetals 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 measuredtowards 25 stars in our Galaxy. We find large variations in metallicity over afactor 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 possiblybelow. Pristine gas falling onto the disk in the form of high-velocity cloudscan cause the observed chemical inhomogeneities on scales of tens of pc. Ourresults suggest that this low-metallicity accreting gas does not efficientlymix into the ISM, which may help us understand metallicity deviations in nearbycoeval stars.

Large Metallicity Variations in the Galactic Interstellar Medium