Unstable area means a location that is susceptible to natural or human-induced events or forces capable of impairing the integrity, including structural components of some or all of the CCR unit that are responsible for preventing releases from such unit. Unstable areas can include poor foundation conditions, areas susceptible to mass movements, and karst terrains.
40 CFR § 257.53
The following definitions apply to this subpart. Terms not defined in this section have the meaning given by RCRA.