  We investigate the prospects for the past or current existence of habitable
conditions deep underneath the surfaces of the Moon and Mars as well as generic
bound and free-floating extrasolar rocky objects. We construct a simple model
that takes into account the thermal limits of life as well as the size, surface
temperature, and relative radionuclide abundance of a given object and yields
the spatial extent of the subsurface habitable region. We also investigate the
constraint imposed by pressure on habitability, and show that it is unlikely to
rule out the prospects for life altogether. We estimate the maximum biomass
that might be sustainable in deep subsurface environments as a function of the
aforementioned parameters from an energetic perspective. We find that it might
be a few percent that of Earth's subsurface biosphere, and three orders of
magnitude smaller than Earth's global biomass, under ideal circumstances. We
conclude with a brief exposition of the prevalence of rocky objects with deep
biospheres and methods for detecting signatures of biological activity through
forthcoming missions to visit the Moon and Mars.
