  A compact source, G0.02467-0.0727, was detected in ALMA \threemm observations
in continuum and very broad line emission. The continuum emission has a
spectral index $\alpha\approx3.3$, suggesting that the emission is from dust.
The line emission is detected in several transitions of CS, SO, and SO$_2$ and
exhibits a line width FWHM $\approx160$ \kms. The line profile appears
Gaussian. The emission is weakly spatially resolved, coming from an area on the
sky $\lesssim1"$ in diameter ($\lesssim10^4$ AU at the distance of the Galactic
Center; GC). The centroid velocity is $v_{LSR}\approx40$-$50$ \kms, which is
consistent with a location in the Galactic Center. With multiple SO lines
detected, and assuming local thermodynamic equilibrium (LTE) conditions,
$T_\mathrm{LTE} = 13$ K, which is colder than seen in typical GC clouds, though
we cannot rule out low-density, subthermally excited, warmer gas. Despite the
high velocity dispersion, no emission is observed from SiO, suggesting that
there are no strong ($\gtrsim10~\mathrm{km~s}^{-1}$) shocks in the molecular
gas. There are no detections at other wavelengths, including X-ray, infrared,
and radio.
  We consider several explanations for the Millimeter Ultra-Broad Line Object
(MUBLO), including protostellar outflow, explosive outflow, collapsing cloud,
evolved star, stellar merger, high-velocity compact cloud, intermediate mass
black hole, and background galaxy. Most of these conceptual models are either
inconsistent with the data or do not fully explain it. The MUBLO is, at
present, an observationally unique object.
