  [Abridged] We unveil the complex shape of a proto-supercluster at z~2.45 in
the COSMOS field using the spectroscopic redshifts of the VIMOS Ultra-Deep
Survey (VUDS), complemented by the zCOSMOS-Deep sample and high-quality
photometric redshifts. The method relies on a 2D Voronoi tessellation in
overlapping redshift slices that is converted into a 3D density field. The
galaxy distribution is constructed using a statistical treatment of
spectroscopic and photometric redshifts. We identify a proto-supercluster,
dubbed "Hyperion" for its immense size and mass, which extends over a volume of
~60x60x150 comoving Mpc^3 and has an estimated total mass of ~4.8x 10^15 M_sun.
This immensely complex structure contains at least 7 density peaks within 2.4 <
z < 2.5, connected by filaments. Based on the peaks average matter density, we
estimate their total mass, M_tot, and find a range of masses from ~0.1x10^14
M_sun to ~2.7x10^14 M_sun. By using spectroscopic members of each peak, we
obtain the velocity dispersion of the galaxies in the peaks, and then their
virial mass M_vir (under the strong assumption that they are virialised). The
agreement between M_vir and M_tot is surprisingly good, considering that almost
all the peaks are probably not yet virialised. According to the spherical
collapse model, the peaks have already started or are about to start
collapsing, and they are all predicted to be virialised by redshift z~0.8-1.6.
We finally perform a careful comparison with the literature, given that smaller
components of this proto-supercluster had previously been identified using
heterogeneous galaxy samples. With VUDS, we obtain a panoramic view of this
large structure, that encompasses, connects, and considerably expands in a
homogeneous way on all previous detections of the various sub-components. This
provides us the unique possibility to study a rich supercluster in formation.
