  Intrinsically explosive growth of a ballooning finger is demonstrated in
nonlinear magnetohydrodynamic calculations of high-$\beta$ disruptions in
tokamaks. The explosive finger is formed by an ideally unstable n=1 mode,
dominated by an m/n=2/1 component. The quadrupole geometry of the 2/1 perturbed
pressure field provides a generic mechanism for the formation of the initial
ballooning finger and its subsequent transition from exponential to explosive
growth, without relying on secondary processes. The explosive ejection of the
hot plasma from the core and stochastization of the magnetic field occur in
Alfv\'enic time scales, accounting for the extremely fast growth of the
precursor oscillations and the rapidity of the thermal quench in some
high-$\beta$ disruptions.
