  We present the first application of a spherical collapse model to a
supercluster of galaxies. Positions and redshifts of $\sim 3000$ galaxies in
the Shapley Supercluster (SSC) are used to define velocity caustics that limit
the gravitationally collapsing structure in its central part. This is found to
extend at least to $8 h^{-1}$ Mpc of the central cluster, A 3558, enclosing 11
ACO clusters. Infall velocities reach $\sim 2000$ km s$^{-1}$. Dynamical models
of the collapsing region are used to estimate its mass profile. An upper bound
on the mass, based on a pure spherical infall model, gives $M(<8 h^{-1}{\rm
Mpc}) ^{<}_{\sim} 1.3\times 10^{16}h^{-1}M_\odot$ for an Einstein-de Sitter
(critical) Universe and $M(<8 h^{-1}{\rm Mpc}) ^{<}_{\sim} 8.5\times
10^{15}h^{-1}M_\odot$ for an empty Universe. The model of Diaferio & Geller
(1997), based on estimating the escape velocity, gives a significantly lower
value, $M(<8 h^{-1}{\rm Mpc})\approx 2.1\times 10^{15}h^{-1}M_\odot$, very
similar to the mass found around the Coma cluster by the same method (Geller et
al. 1999), and comparable to or slightly lower than the dynamical mass in the
virialized regions of clusters enclosed in the same region of the SSC. In both
models, the overdensity in this region is substantial, but far from the value
required to account for the peculiar motion of the Local Group with respect to
the cosmic microwave background.
