Tokamak stability to, potentially explosive, `ballooning' displacements of
elliptical magnetic flux tubes is examined in large aspect ratio equilibrium.
Above a critical pressure gradient the energy stored in the plasma may be
lowered by finite (but not infinitesimal) displacements of such tubes
(metastability). Above a higher pressure gradient, the linear stability
boundary, such tubes are linearly and nonlinearly unstable. The flux tube
displacement can be of the order of the pressure gradient scale length. Plasma
transport from displaced flux tubes may result in rapid loss of confinement.