  The Southern Coalsack is located in the interior of the Upper Centaurus-Lupus
(UCL) super bubble and shows many traits that point to a much more energetic
environment than might be expected from a dark, starless molecular cloud. A
hot, X-ray emitting, envelope surrounds the cloud, it has a very strong
internal magnetic field and its darkest core seems to be on astronomical time
scales "just about" to start forming stars. In order to probe the magnetic
environment of the cloud and to compare with the optical/near infrared
polarimetry-based field estimates for the cloud, we have acquired Faraday
Rotation measurements towards the pulsar PSR J1210$-$6550, probing the magnetic
field in the vicinity of the cloud, and a comparison target, PSR J1435$-$5954,
at a similar line of sight distance but several degrees from the cloud. Both
lines of sight hence primarily probe the UCL super bubble. The earlier
estimates of the magnetic field inside the Coalsack, using the
Chandrasekhar-Fermi method on optical and near-infrared polarimetry, yield
B$_\perp$ = 64--93 $\mu$G. However, even though PSR J1210$-$6550 is located
only $\sim$30 arc minutes from the (CO) edge of the cloud, the measured field
strength is only B$_\parallel$ = 1.1$\pm$0.2 $\mu$G. While thus yielding a very
high field contrast to the cloud we argue that this might be understood as due
to the effects on the cloud by the super bubble.
