  Low-frequency radio observations of neutral hydrogen during and before the
epoch of cosmic reionization will provide ~ 1000 quasi-independent source
planes, each of precisely known redshift, if a resolution of ~1 arcminutes or
better can be attained. These planes can be used to reconstruct the projected
mass distribution of foreground material. Structure in these source planes is
linear and gaussian at high redshift (30<z<300) but is nonlinear and
nongaussian during re ionization. We demonstrate that this structure can, in
principle, be used to make mass images with a formal signal-to-noise per pixel
exceeding 10, even for pixels as small as an arc-second. With an ideal
telescope, both resolution and signal-to-noise can exceed those of even the
most optimistic idealized mass maps from galaxy lensing by more than an order
of magnitude. Individual dark halos similar in mass to that of the Milky Way
could be imaged with high signal-to-noise out to z ~ 10. Even with a much less
ambitious telescope, a wide-area survey of 21 cm lensing would provide very
sensitive constraints on cosmological parameters, in particular on dark energy.
These are up to 20 times tighter than the constraints obtainable from
comparably sized, very deep surveys of galaxy lensing, although the best
constraints come from combining data of the two types. Any radio telescope
capable of mapping the 21cm brightness temperature with good frequency
resolution (~ 0.05 MHz) over a band of width >~ 10 MHz should be able to make
mass maps of high quality. The planned Square Kilometer Array (SKA) may be able
to map the mass with moderate signal-to-noise down to arcminute scales,
depending on the reionization history of the universe and the ability to
subtract foreground sources.
