  SPT0346-52 is one of the most most luminous and intensely star-forming
galaxies in the universe, with L_FIR > 10^13 L_sol and Sigma_SFR ~ 4200 M_sol
yr^-1 kpc^-2. In this paper, we present ~0.15'' ALMA observations of the
[CII]158micron emission line in this z=5.7 dusty star-forming galaxy. We use a
pixellated lensing reconstruction code to spatially and kinematically resolve
the source-plane [CII] and rest-frame 158 micron dust continuum structure at
~700 pc (~0.12'') resolution. We discuss the [CII] deficit with a pixellated
study of the L_[CII]/L_FIR ratio in the source plane. We find that individual
pixels within the galaxy follow the same trend found using unresolved
observations of other galaxies, indicating that the deficit arises on scales
<700 pc. The lensing reconstruction reveals two spatially and kinematically
separated components (~1 kpc and ~500 km s^-1 apart) connected by a bridge of
gas. Both components are found to be globally unstable, with Toomre Q
instability parameters << 1 everywhere. We argue that SPT0346-52 is undergoing
a major merger, which is likely driving the intense and compact star formation.
