  The results of deep long-slit spectroscopy of the extremely low-metallicity
blue compact dwarf (BCD) galaxy SBS 0335-052 are presented. Down to intensity
levels of 10^{-3 ... -4} of Hbeta, unprecedented for spectroscopy of
extra-galactic giant H II regions, we detect numerous weak permitted and
forbidden nebular lines in the brightest part of the galaxy. With varying
degrees of confidence, the detections include lines of high-ionization ions
like Fe^{4+} -Fe^{6+}, implying very hard ionizing radiation. Two broad
emission features, possibly from Wolf-Rayet stars, and stellar He II 4200
absorption are seen in the same region. The large spatial extent of He II 4686
emission (implying the presence of sufficient ionizing photons with energies
above 54 eV) and the spatial distribution of the electron temperature suggest
that at least some part of the hard radiation is associated with shocks.
Extended Halpha emission is detected over ~ 6 - 8 kpc, a much larger area than
in previous studies, suggesting that hot ionized gas is spread out far away
from the central ionizing clusters. This shows that nebular line and continuous
emission can significantly modify the colours of these extended regions and
must be taken into account in studies of the underlying stellar population.
