  We use a 27.6 deg$^2$ survey to measure the clustering of $gzK_s$-selected
quiescent galaxies at $z\sim1.6$, focusing on ultra-massive quiescent galaxies.
We find that $z\sim1.6$ Ultra-Massive Passively Evolving Galaxies (UMPEGs),
which have $K_s(AB)<19.75$ (stellar masses of $M_{stars}$ $>\sim
10^{11.4}M_{\odot}$ and mean $<$$M_{stars}$$>$ = $10^{11.5}M_{\odot}$), cluster
more strongly than any other known galaxy population at high redshift.
Comparing their correlation length, $r_0 = 29.77 \pm 2.75$ $ h^{-1}$Mpc, with
the clustering of dark matter halos in the Millennium XXL N-body simulation
suggests that these $z\sim1.6$ UMPEGs reside in dark matter halos of mass
$M_{h}\sim10^{14.1}h^{-1}M_{\odot}$. Such very massive $z\sim1.6$ halos are
associated with the ancestors of $z\sim0$ massive galaxy clusters such as the
Virgo and Coma clusters. Given their extreme stellar masses and lack of
companions with comparable mass, we surmise that these UMPEGs could be the
already-quenched central massive galaxies of their (proto)clusters. We conclude
that with only a modest amount of further growth in their stellar mass,
$z\sim1.6$ UMPEGs could be the progenitors of some of the massive central
galaxies of present-day massive galaxy clusters observed to be already very
massive and quiescent near the peak epoch of the cosmic star formation.
