The rapid, Alfv¿nic timescale of onset has been an enigma ever since magnetospheric substorms were first identified. Investigators have proposed a variety of trigger mechanisms in an effort to account for the abrupt reconfiguration observed. No one mechanism clearly stands out as the single cause of substorms. Recent analysis has demonstrated that field lines in the tail of the magnetosphere are metastable to ballooning type instabilities via a process called nonlinear magnetohydrodynamic detonation. Furthermore, the perturbation amplitude necessary to activate the nonlinear instability is not large. Thus a variety of external and internal perturbations can trigger the same basic nonlinear instability. The evidence for detonation as a medium for the unification of substorm triggers is discussed. ¿ 1999 American Geophysical Union