  A weak-lensing analysis of wide-field $V$- and $I$-band images centered on
the cluster Abell 1942 has uncovered a mass concentration $\sim 7$ arcminutes
South of the cluster center. A statistical analysis shows that the detections
are highly significant. No strong concentration of bright galaxies is seen at
the position of the mass concentration, though a slight galaxy number
overdensity and a weak extended X-ray source are present about 1' away from its
center.
  From the spatial dependence of the tangential alignment around the center of
the mass concentration, we inferred a lower bound on the mass inside a sphere
of radius $0.5 h^{-1}$\ts Mpc of $1\times 10^{14}h^{-1}M_\odot$, much higher
than crude mass estimates based on X-ray data. No firm conclusion can be
inferred about the nature of the clump. If it were a high-redshift cluster, the
weak X-ray flux would indicate that it had an untypically low X-ray luminosity
for its mass; if the X-ray emission were physically unrelated to the mass
concentration, this conclusion would be even stronger.
  The search for massive halos by weak lensing enables us for the first time to
select halos based on their mass properties only and to detect new types of
objects, e.g., dark halos. The mass concentration in the field of A1942 may be
the first example of such a halo.
