  The advent of the JWST has revolutionised our understanding of high-redshift
galaxies. In particular, the NIRCam instrument on-board JWST has revealed a
population of Hubble Space Telescope (HST)-dark galaxies that had previously
evaded optical detection, potentially due to significant dust obscuration,
quiescence, or simply extreme redshift. Here, we present the first NIRSpec
spectra of 23 HST-dark galaxies ($\mathrm{H-F444W>1.75}$), unveiling their
nature and physical properties. This sample includes both dusty and quiescent
galaxies with spectroscopic data from NIRSpec/PRISM, providing accurate
spectroscopic redshifts with $\mathrm{\overline{z}_{spec} = 4.1 \pm 0.7}$. The
spectral features demonstrate that, while the majority of HST-dark galaxies are
dusty, a substantial fraction, $\mathrm{13^{+9}_{-6} \%}$, are quiescent. For
the dusty galaxies, we have quantified the dust attenuation using the Balmer
decrement ($\mathrm{H\alpha / H\beta}$), finding attenuations $\mathrm{A_{V} >
2\ mag}$. We find that HST-dark dusty galaxies are $\mathrm{H\alpha}$ emitters
with equivalent widths spanning the range $\mathrm{ 68 A < EW_{H\alpha} < 550 A
}$, indicative of a wide range of recent star-formation activity. Whether dusty
or quiescent, we find that HST-dark galaxies are predominantly massive, with
85\% of the galaxies in the sample having masses $\mathrm{log(M_{*}/M_{\odot})
> 9.8}$. This pilot NIRSpec program reveals the diverse nature of HST-dark
galaxies and highlights the effectiveness of NIRSpec/PRISM spectroscopic
follow-up in distinguishing between dusty and quiescent galaxies and properly
quantifying their physical properties. Upcoming research utilising
higher-resolution NIRSpec data and combining JWST with ALMA observations will
enhance our understanding of these enigmatic and challenging sources.
