  We chart the expected Galactic distribution of neutron stars and black holes.
These compact remnants of dead stars -- the Galactic underworld -- are found to
exhibit a fundamentally different distribution and structure to the visible
Galaxy. Compared to the visible Galaxy, concentration into a thin flattened
disk structure is much less evident with the scale height more than tripling to
1260 +- 30 pc. This difference arises from two primary causes. Firstly, the
distribution is in part inherited from the integration over the evolving
structure of the Galaxy itself (and hence the changing distribution of the
parent stars). Secondly, an even larger effect arises from the natal kick
received by the remnant at the event of its supernova birth. Due to this kick
we find 30% of remnants have sufficient kinetic energy to entirely escape the
Galactic potential (40% of neutron stars and 2% of black holes) leading to a
Galactic mass loss integrated to the present day of ~ 0.4% of the stellar mass
of the Galaxy. The black hole -- neutron star fraction increases near the
Galactic centre: a consequence of smaller kick velocities in the former (the
assumption made is that kick velocity is inversely proportional to mass). Our
simulated remnant distribution yields probable distances of 19 pc and 21 pc to
the nearest neutron star and black hole respectively, while our nearest
probable magnetar lies at 4.2 kpc. Although the underworld only contains of
order ~ 1% of the Galaxy's mass, observational signatures and physical traces
of its population, such as microlensing, will become increasingly present in
data ranging from gravitational wave detectors to high precision surveys from
space missions such as Gaia.
