  We consider the parabolic-elliptic Keller-Segel system in three dimensions
and higher, corresponding to the mass supercritical case. We construct
rigorously a solution which blows up in finite time by having its mass
concentrating near a ring that shrinks to a point. In particular, the
singularity is of type II, non self-similar. We show the stability of this
dynamics among spherically symmetric solutions. In renormalised variables, the
solution ressembles a traveling wave imploding at the origin, and this, to our
knowledge, is the first stability result for such phenomenon for an evolution
PDE. We develop a framework to handle the interactions between the two blowup
zones contributing to the mechanism: a thin inner zone around the ring where
viscosity effects occur, and an outer zone where the evolution is mostly
inviscid.
