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(Abridged) The initial physical conditions of high-mass stars andprotoclusters remain poorly characterized. To this end we present the firsttargeted ALMA 1.3mm continuum and spectral line survey towards high-massstarless clump candidates, selecting a sample of 12 of the most massivecandidates ($400-4000\, M_\odot$) within 5 kpc. The joint 12+7m array maps havea high spatial resolution of $\sim 3000\, \mathrm{au}$ ($\sim0.8^{\prime\prime}$) and have point source mass-completeness down to $\sim0.3\, M_\odot$ at $6\sigma$ (or $1\sigma$ column density sensitivity of$1.1\times10^{22}\, \mathrm{cm^{-2}}$). We discover previously undetectedsignposts of low-luminosity star formation from CO (2-1) and SiO (5-4) bipolaroutflows and other signatures towards 11 out of 12 clumps, showing that currentMIR/FIR Galactic Plane surveys are incomplete to low- and intermediate-massprotostars ($\lesssim 50\, L_\odot$). We compare a subset of the observed coreswith a suite of radiative transfer models of starless cores. We find ahigh-mass starless core candidate with a model-derived mass consistent with$29^{52}_{15}\, M_\odot$ when integrated over size scales of $2\times10^4\,\mathrm{au}$. Unresolved cores are poorly fit by starless core models,supporting the interpretation that they are protostellar even without detectionof outflows. Substantial fragmentation is observed towards 10 out of 12 clumps.We extract sources from the maps using a dendrogram to study the characteristicfragmentation length scale. Nearest neighbor separations when corrected forprojection are consistent with being equal to the clump average thermal Jeanslength. Our findings support a hierarchical fragmentation process, where thehighest density regions are not strongly supported against thermalgravitational fragmentation by turbulence or magnetic fields.

ALMA observations of fragmentation, sub-structure, and protostars in high-mass starless clump candidates