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tokens: elemental-abundance ratios ([X/Fe]) were derived with the BACCHUS code. Table 1 Atmospheric Parameters and Elemental Abundances of Stars in VVV CL001 Note. The errors were determined in the same manner as described in Fernández-Trincado et al. ( 2020a ) by varying the atmospheric parameters one at a time by the typical, albeit conservative, values of Δ T eff = ±100 K, g = ±100 cgs, and Δ ξ t = ±0.05 km s −1 . Thus, the reported uncertainties are defined as . Figure 2 shows the good quality of the APOGEE-2 spectrum compared to the best-fit synthetic spectra for selected atomic and molecule lines of the two stars in VVV CL001, from which the chemical-abundance ratios were determined. For each spectrum we were able to identify reliable lines for six chemical species (N, O, Mg, Al, Si, and Fe). The same figure also reveals the scarcely detectable Fe i lines. The resulting elemental-abundance ratios are listed in Table 1 . Figure 2. Detection of atomic and molecular lines. Spectral synthesis is shown for the determination of the [N/Fe], [O/Fe], [Mg/Fe], [Al/Fe], [Si/Fe], and [Fe/H] abundances for two stars in the innermost regions of VVV CL001. Each panel shows the best-fit syntheses (red line) from BACCHUS compared to the observed spectra (black squares) of selected lines (black arrows). The resulting chemical abundances are also displayed in Figure 3 , and compared to four GCs at similar metallicity taken from the APOGEE-2 GC sample of Mészáros et al. ( 2020 ). We find that the two program stars exhibit an iron abundance ratio of −2.47 to −2.44, suggesting that VVV CL001 has a mean metallicity [Fe/H] = −2.45, with an uncertainty due to systematics of 0.24 dex, which makes this cluster possibly the most metal-poor GC identified so far within the Sun’s Galactocentric distance, and with an extreme metallicity close to the apparent “floor” in the empirical metallicity distribution function for GCs in the MW and Local Universe (e.g., Geisler et al. 1995 ; Simpson 2018 ; Kruijssen 2019 ; Larsen et al. 2020 ; Wan et al. 2020 ). Note that VVV CL001 has the lowest Fe abundance among this sample, which includes the lowest metallicity GCs observed by APOGEE. With our limited sample, we do not find evidence of an intrinsic Fe-abundance spread. Figure 3. Elemental abundance of stars in VVV CL001 (black symbols) and four comparison metal-poor GCs. Each violin representation show the univariate kernel density estimate of the abundance ratios of each cluster as determined from Mészáros et al. ( 2020 ). Each GC has been slightly offset horizontally to distinguish them. Regarding the α -elements (O, Mg, and Si), VVV CL001 displays a modest α -element enhancement, a clear signature of the fast enrichment provided by SNe II events, and compatible with other Galactic metal-poor GCs at similar metallicity (see Figure 3 ). Furthermore, no Mg–Al anticorrelation is evident in our small sample; the two VVV CL001 stars exhibit an aluminum deficit, which places them within