The Enigmatic (Almost) Dark Galaxy Coma P: The Atomic Interstellar
  Medium
  We present new high-resolution HI spectral line imaging of Coma P, the
brightest HI source in the system HI 1232$+$20. This extremely low surface
brightness galaxy was first identified in the ALFALFA survey as an "(Almost)
Dark" object: a clearly extragalactic HI source with no obvious optical
counterpart in existing optical survey data (although faint ultraviolet
emission was detected in archival GALEX imaging). Using a combination of data
from the Westerbork Synthesis Radio Telescope and the Karl G. Jansky Very Large
Array, we investigate the HI morphology and kinematics at a variety of physical
scales. The HI morphology is irregular, reaching only moderate maxima in mass
surface density (peak $\sigma_{\rm HI}\sim 10$ $M_{\odot}$ pc$^{-2}$). Gas of
lower surface brightness extends to large radial distances, with the HI
diameter measured at 4.0$\pm$0.2 kpc inside the 1 $M_{\odot}$ pc$^{-2}$ level.
We quantify the relationships between HI gas mass surface density and various
types of star formation by considering GALEX far ultraviolet observations and
H$\alpha$ nondetections. We describe Coma P's complex HI kinematics using
spatially resolved position-velocity analysis and three-dimensional modeling.
Both methods of analysis suggest that Coma P's kinematics show signatures of
either the collision of two HI disks or a significant infall event. Coma P is
just consistent (within 3$\sigma$) with the known M$_{\rm HI}$ -- D$_{\rm HI}$
scaling relation. It is either too large for its HI mass, has too low an HI
mass for its HI size, or the two HI components artificially extend its HI size.
Coma P lies within the empirical scatter at the faint end of the baryonic
Tully--Fisher relation, although the complexity of the HI dynamics complicates
the interpretation. The collective HI characteristics of Coma P make it unusual
among known galaxies in the nearby universe. [Abridged]
