  We present precision Penning-trap mass measurements of neutron-rich calcium
and potassium isotopes in the vicinity of neutron number N=32. Using the TITAN
system the mass of $^{51}$K was measured for the first time, and the precision
of the $^{51,52}$Ca mass values were improved significantly. The new mass
values show a dramatic increase of the binding energy compared to those
reported in the atomic mass evaluation. In particular, $^{52}$Ca is more bound
by 1.74 MeV, and the behavior with neutron number deviates substantially from
the tabulated values. An increased binding was predicted recently based on
calculations that include three-nucleon (3N) forces. We present a comparison to
improved calculations, which agree remarkably with the evolution of masses with
neutron number, making neutron-rich calcium isotopes an exciting region to
probe 3N forces at neutron-rich extremes.
