  We report the discovery of a giant dust scattering ring around the Black Hole
transient MAXI J1348-630 with SRG/eROSITA during its first X-ray all-sky
survey. During the discovery observation in February 2020 the ring had an outer
diameter of 1.3 deg, growing to 1.6 deg by the time of the second all sky
survey scan in August 2020. This makes the new dust ring the by far largest
X-ray scattering ring observed so far. Dust scattering halos, in particular the
rings found around transient sources, offer the possibility of precise distance
measurements towards the original X-ray sources. We combine data from
SRG/eROSITA, XMM-Newton, MAXI, and Gaia to measure the geometrical distance of
MAXI J1348-630. The Gaia data place the scattering dust at a distance of 2050
pc, from the measured time lags and the geometry of the ring, we find MAXI
J1348-630 at a distance of 3390 pc with a statistical uncertainty of only 1.1%
and a systematic uncertainty of 10% caused mainly by the parallax offset of
Gaia. This result makes MAXI J1348-630 one of the black hole transients with
the best determined distances. The new distance leads to a revised mass
estimate for the black hole of 11+-2 solar masses, the transition to the soft
state during the outburst occurred when the bolometric luminosity of MAXI
J1348-630 had reached 1.7% of its Eddington luminosity.
