  Cosmic voids, the underdense regions in the universe, are particularly
sensitive to diffuse density components such as cosmic neutrinos. This
sensitivity is enhanced by the match between void sizes and the free-streaming
scale of massive neutrinos. Using the massive neutrino simulations
\texttt{MassiveNuS}, we investigate the effect of neutrino mass on dark matter
halos as a function of environment. We find that the halo mass function depends
strongly on neutrino mass and that this dependence is more pronounced in voids
than in high-density environments. An observational program that measured the
characteristic mass of the most massive halos in voids should be able to place
novel constraints on the sum of the masses of neutrinos $\sum m_\nu$. The
neutrino mass effect in the simulations is quite strong: In a 512$^3$ $h^{-3}$
Mpc$^3$ survey, the mean mass of the 1000 most massive halos in the void
interiors is $(4.82 \pm 0.11) \times 10^{12} h^{-1}M_{\odot}$ for $\sum m_\nu =
0.6$ eV and $(8.21 \pm 0.13) \times 10^{12} h^{-1}M_{\odot}$ for $\sum m_\nu =
0.1$ eV. Subaru (SuMIRe), Euclid and WFIRST will have both spectroscopic and
weak lensing surveys. Covering volumes at least 50 times larger than our
simulations, they should be sensitive probes of neutrino mass through void
substructure.
