  Starlight polarimetry, when combined with accurate distance measurements,
allows for exploration of the three-dimensional structure of local magnetic
fields in great detail. We present optical polarimetric observations of stars
in and close to the Southern Coalsack, taken from the Interstellar Polarization
Survey (IPS). Located in five fields of view approximately $0.3^{o}$ by
$0.3^{o}$ in size, these data represent the highest density of optical
polarimetric observations in the Southern Coalsack to date. Using these data,
combined with accurate distances and extinctions based on Gaia data, we are
able to characterize the magnetic field of the Coalsack and disentangle
contributions to the polarization caused by the Southern Coalsack and a
background structure. For the Southern Coalsack, we find an average magnetic
field orientation of $\theta\sim 75^{o}$ with respect to the Galactic north
pole and an average plane-of-sky magnetic field strength of approximately
$B_{POS}=10$ $\mu G$, using the Davis-Chandrasekhar-Fermi (DCF) method. These
values are in agreement with some earlier estimates of the Coalsack's magnetic
field. In order to study the distant structure, we introduce a simple method to
separate and isolate the polarization of distant stars from foreground
contribution. For the distant structure, which we estimate to be located at a
distance of approximately 1.3-1.5 kpc, we find an average magnetic field
orientation of $\theta\sim100^{o}$ and we estimate a field strength of
$B_{POS}\sim10 \ \mu G$, although this will remain highly uncertain until the
precise nature of the distant structure can be uncovered.
