  Carbon disulfide, CS$_2$, is an archetypal double-bonded molecular system
belonging to the rich class of group IV-group VI, AB$_2$ compounds. It is
widely and since long time believed that upon compression at several GPa a
polymeric chain of type (-(C=S)-S-)$_n$ named Bridgman's black polymer will
form. By combining optical spectroscopy and synchrotron X-ray diffraction data
with ab initio simulations, we demonstrate that the structure of the Bridgman's
black polymer is remarkably different. Solid molecular CS$_2$ undergoes a
pressure-induced structural transformation at around 10-11 GPa, developing a
disordered polymeric system. The polymer consists of 3-fold and 4-fold
coordinated carbon atoms with an average carbon coordination continuously
increasing upon further compression to 40 GPa. Polymerization also gives rise
to some C=C double bonds. Upon decompression, the structural changes are
partially reverted, a very small amount of molecular CS$_2$ is recovered, while
the sample undergoes partial chemical disproportionation. Our work uncovers the
non-trivial high-pressure structural evolution in one of the simplest molecular
systems exhibiting molecular as well as polymeric phases.
