The Three-mm Ultimate Mopra Milky Way Survey. I. Survey Overview,
  Initial Data Releases, and First Results
  We describe a new mm-wave molecular-line mapping survey of the southern
Galactic Plane and its first data releases. The Three-mm Ultimate Mopra Milky
Way Survey (ThrUMMS) maps a 60{\deg}x2{\deg} sector of our Galaxy's fourth
quadrant, using a combination of fast mapping techniques with the Mopra radio
telescope, simultaneously in the J=1-0 lines of $^{12}$CO, $^{13}$CO,
C$^{18}$O, and CN near 112 GHz at ~arcminute and ~0.3 km s$^{-1}$ resolution,
with ~2 K channel$^{-1}$ sensitivity for $^{12}$CO and ~1 K channel$^{-1}$ for
the other transitions. The calibrated data cubes from these observations are
made available to the community after processing through our pipeline. Here, we
describe the motivation for ThrUMMS, the development of new observing
techniques for Mopra, and how these techniques were optimised to the objectives
of the survey. We showcase some sample data products and describe the first
science results on CO-isotopologue line ratios. These vary dramatically across
the Galactic Plane, indicating a very wide range of optical depth and
excitation conditions, from warm and translucent to cold and opaque. The
population of cold clouds in particular have optical depths for $^{12}$CO
easily exceeding 100. We derive a new, nonlinear conversion law from $^{12}$CO
integrated intensity to column density, which suggests that the molecular mass
traced by CO in the Galactic disk may have been substantially underestimated.
This further suggests that some global relationships in disk galaxies, such as
star formation laws, may need to be recalibrated. The large ThrUMMS team is
proceeding with several other science investigations.
