  We present a physical characterization of MMJ100026.36+021527.9 (a.k.a.
``MAMBO-9''), a dusty star-forming galaxy (DSFG) at $z=5.850\pm0.001$. This is
the highest redshift unlensed DSFG (and fourth most distant overall) found
to-date, and is the first source identified in a new 2mm blank-field map in the
COSMOS field. Though identified in prior samples of DSFGs at 850$\mu$m-1.2mm
with unknown redshift, the detection at 2mm prompted further follow-up as it
indicated a much higher probability that the source was likely to sit at $z>4$.
Deep observations from the Atacama Large Millimeter and submillimeter Array
(ALMA) presented here confirm the redshift through the secure detection of
$^{12}$CO($J\!=$6$\rightarrow$5) and
p-H$_{2}$O(2$_{1,1}\!\rightarrow$2$_{0,2}$). MAMBO-9 is comprised of a pair of
galaxies separated by 6kpc with corresponding star-formation rates of
590M$_\odot$yr$^{-1}$ and 220M$_\odot$yr$^{-1}$ total molecular hydrogen gas
mass of (1.7$\pm$0.4)$\times10^{11}$M$_\odot$, dust mass of
(1.3$\pm$0.3)$\times10^{9}$M$_\odot$ and stellar mass of
(3.2$^{+1.0}_{-1.5}$)$\times10^{9}$M$_\odot$. The total halo mass,
(3.3$\pm$0.8)$\times10^{12}$M$_\odot$, is predicted to exceed
$>10^{15}$M$_\odot$ by $z=0$. The system is undergoing a merger-driven
starburst which will increase the stellar mass of the system tenfold in
$\tau_{\rm depl}=40-80$Myr, converting its large molecular gas reservoir (gas
fraction of 96$^{+1}_{-2}$%) into stars. MAMBO-9 evaded firm spectroscopic
identification for a decade, following a pattern that has emerged for some of
the highest redshift DSFGs found. And yet, the systematic identification of
unlensed DSFGs like MAMBO-9 is key to measuring the global contribution of
obscured star-formation to the star-formation rate density at $z>4$, the
formation of the first massive galaxies, and the formation of interstellar dust
at early times ($<$1Gyr).
