  Stellar ejecta gradually enrich the gas out of which subsequent stars form,
making the least chemically enriched stellar systems direct fossils of
structures formed in the early universe. Although a few hundred stars with
metal content below one thousandth of the solar iron content are known in the
Galaxy, none of them inhabit globular clusters, some of the oldest known
stellar structures. These show metal content of at least ~0.2 percent of the
solar metallicity ([Fe/H] > -2.7). This metallicity floor appears universal and
it has been proposed that proto-galaxies that merge into the galaxies we
observe today were simply not massive enough to form clusters that survived to
the present day. Here, we report the discovery of a stellar stream, C-19, whose
metallicity is less than 0.05 per cent the solar metallicity ([Fe/H]=-3.38 +/-
0.06 (stat.) +/- 0.20 (syst.)). The low metallicity dispersion and the chemical
abundances of the C-19 stars show that this stream is the tidal remnant of the
most metal-poor globular cluster ever discovered, and significantly below the
purported metallicity floor: clusters with significantly lower metallicities
than observed today existed in the past and contributed their stars to the
Milky Way halo.
