  We present the discovery of `A Big Ring on the Sky' (BR), the second
ultra-large large-scale structure (uLSS) found in MgII-absorber catalogues,
following the previously reported Giant Arc (GA). In cosmological terms the BR
is close to the GA - at the same redshift $z \sim 0.8$ and with a separation on
the sky of only $\sim 12^\circ$. Two extraordinary uLSSs in such close
configuration raises the possibility that together they form an even more
extraordinary cosmological system. The BR is a striking circular, annulus-like,
structure of diameter $\sim 400$ Mpc (proper size, present epoch). The method
of discovery is as described in the GA paper, but here using the new
MgII-absorber catalogues restricted to DR16Q quasars. Using the Convex Hull of
Member Spheres (CHMS) algorithm, we estimate that the annulus and inner
absorbers of the BR have departures from random expectations, at the density of
the control field, of up to $5.2\sigma$. We present the discovery of the BR,
assess its significance using the CHMS, Minimal Spanning Tree (MST), FilFinder
and Cuzick & Edwards (CE) methods, show it in the context of the GA+BR system,
and suggest some implications for the origins of uLSS and for our understanding
of cosmology. For example, it may be that unusual geometric patterns, such as
these uLSSs, have an origin in cosmic strings.
