  It is demonstrated that a straightforward extension of the Arrhenius law
accurately describes diffusion in the thermodynamically stable liquid of hard
spheres. A sharp negative deviation from this behaviour is observed as the
liquid is compressed beyond its stability limit, which can be compared with
super-Arrhenius slowing down in supercooled liquids. It is suggested that the
observed dynamical transition is caused by extra entropy barriers arising in
the free-energy landscape. This interpretation is corroborated by the
observation of characteristic anomalies in the geometry of a particle
trajectory.
