  We present HST observations that show a bifurcation of colors in the middle
main sequence of the globular cluster Omega Centauri. We see this in three
different fields, observed with different cameras and filters. We also present
high precision photometry of a central ACS field which shows a number of
main-sequence turnoffs and subgiant branches. The double main sequence, the
multiple turnoffs and subgiant branches, and other population sequences
discovered in the past along the red giant branch of this cluster add up to a
fascinating but frustrating puzzle. We suggest various explanations, none of
them very conclusive.
