  The X-ray sky is not as well known as is sometimes thought. We report on our
search of minority populations (Kim & Elvis 1999). One of the most intriguing
is that of `blank field sources', i.e. bright ROSAT sources (F(X)>1e-13
erg/cm2/s) with no optical counterpart on the Palomar Sky Survey (to O=21.5)
within their 39"(99%) radius error circle. The nature of Blank Field sources is
unknown: no known extragalactic population has such extreme X-ray to optical
ratios (f(X)/f(V) >60). Moreover blank field source tend to have much flatter
PSPC spectra compared to radio-quiet AGN. Both properties suggest obscuration.
`Blank field sources' could be: Quasar-2s, low-mass AGNs, isolated neutron
stars, high redshift clusters of galaxies, failed clusters, AGNs with no big
blue bump. Identification with any of these populations would be an interesting
discovery.
