The mechanisms by which ion tracks are produced are subject to some debate. They can be considered to produce thermal spikes in the sense that they lead to strong lattice heating and a transient disordered atom zone. However, at least the initial stage of the damage might be better understood in terms of a Coulomb explosion mechanism. Regardless of what the heating mechanism is, it is well established that swift heavy ions typically produce a long nearly cylindrical track of damage in insulators, which has been shown to be underdense in the middle, at least in SiO 2.