  The discoveries of a number of binary black hole mergers by LIGO and VIRGO
has reinvigorated the interest that primordial black holes (PBHs) of tens of
solar masses could contribute non-negligibly to the dark matter energy density.
Should even a small population of PBHs with masses $\gtrsim
\mathcal{O}(M_\odot)$ exist, they could profoundly impact the properties of the
intergalactic medium and provide insight into novel processes at work in the
early Universe. We demonstrate here that observations of the 21cm transition in
neutral hydrogen during the epochs of reionization and cosmic dawn will likely
provide one of the most stringent tests of solar mass PBHs. In the context of
21cm cosmology, PBHs give rise to three distinct observable effects:
${\textit{(i)}}$ the modification to the primordial power spectrum (and thus
also the halo mass function) induced by Poisson noise, ${\textit{(ii)}}$ a
uniform heating and ionization of the intergalactic medium via X-rays produced
during accretion, and ${\textit{(iii)}}$ a local modification to the
temperature and density of the ambient medium surrounding isolated PBHs. Using
a four-parameter astrophysical model, we show that experiments like SKA and
HERA could potentially improve upon existing constraints derived using
observations of the cosmic microwave background by more than one order of
magnitude.
