  Using the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph
(MODS), we have obtained optical spectroscopy of one of the most metal-poor
dwarf star-forming galaxies (SFG) in the local Universe, J2229+2725. This
galaxy with a redshift z=0.0762 was selected from the Data Release 16 (DR16) of
the Sloan Digital Sky Survey (SDSS). Its properties derived from the LBT
observations are most extreme among SFGs in several ways. Its oxygen abundance
12+logO/H=7.085+/-0.031 is among the lowest ever observed for a SFG. With its
very low metallicity, an absolute magnitude Mg=-16.39 mag, a low stellar mass
Mstar=9.1x10^6 Msun and a very low mass-to-light ratio Mstar/Lg~0.0166 (in
solar units), J2229+2725 deviates strongly from the luminosity-metallicity
relation defined by the bulk of the SFGs in the SDSS. J2229$+$2725 has a very
high specific star-formation rate sSFR~75 Gyr^-1, indicating very active
ongoing star formation. Three other features of J2229+2725 are most striking,
being the most extreme among lowest-metallicity SFGs: 1) a ratio
O32=I([OIII]5007)/I([OII]3727)~53, 2) an equivalent width of the Hbeta emission
line EW(Hbeta) of 577A, and 3) an electron number density of ~1000 cm^-3. These
properties imply that the starburst in J2229+2725 is very young. Using the
extremely high O32 in J2229+2725, we have improved the strong-line calibration
for the determination of oxygen abundances in the most metal-deficient
galaxies, in the range 12 + logO/H<7.3.
