  We apply a new non-parametric technique to reconstruct, with uncertainties,
the projected mass distribution of the inner region of Abell 2218, using
combined strong and weak lensing constraints from multiple-image systems and
arclets with known redshifts. The reconstructed mass map broadly resembles
previous, less detailed, parametric models, but when examined in detail shows
several sub-structures, not necessarily associated with light but strongly
required by the lensing data. In particular, the highest mass peak is offset by
about 30 kpc/h50 from the main light peak, and projected mass-to-light in the
directions of different cluster galaxies varies by at least a factor of 10. On
comparing with mass estimates from models of the X-ray emitting gas, we find
that the X-ray models under-predict the enclosed mass profile by, at least a
factor of 2.5; the discrepancy gets worse if we assume that mass traces light
to the extent allowed by the lensing constraints.
