  The Lynx arc, with a redshift of 3.357, was discovered during spectroscopic
follow-up of the z=0.570 cluster RX J0848+4456 from the ROSAT Deep Cluster
Survey. The arc is characterized by a very red R-K color and strong, narrow
emission lines. Analysis of HST WFPC2 imaging and Keck optical and infrared
spectroscopy shows that the arc is an \hii galaxy magnified by a factor of ~10
by a complex cluster environment. The high intrinsic luminosity, the emission
line spectrum, the absorption components seen in Ly\alpha and C IV, and the
restframe ultraviolet continuum are all consistent with a simple HII region
model containing ~ 10^6 hot O stars. The best fit parameters for this model
imply a very hot ionizing continuum (T_BB ~ 80,000 K), high ionization
parameter (log U ~ -1), and low nebular metallicity (Z / Z_\odot ~ 0.05). The
narrowness of the emission lines requires a low mass-to-light ratio for the
ionizing stars, suggestive of an extremely low metallicity stellar cluster. The
apparent overabundance of silicon in the nebula could indicate enrichment by
past pair instability supernov\ae, requiring stars more massive than ~140
M_\odot.
