260 U.S. 393 (U.S. Supreme Court 1922)
Pennsylvania Coal v. Mahon is the case that launched modern regulatory takings doctrine.
Does a state statute prohibiting coal mining that would cause subsidence of surface structures—thereby destroying the value of a reserved mineral estate and nullifying an express waiver of support—effect a taking of property requiring just compensation under the Fifth Amendment as applied to the states through the Fourteenth Amendment?
While the state may regulate property under its police power to protect public health, safety, and welfare, a regulation that goes too far in diminishing private property rights will be recognized as a taking requiring just compensation. Factors relevant to this determination include the extent of economic diminution, the character of the governmental action (including whether it prevents a noxious use or confers general reciprocity of advantage), and the degree to which the regulation singles out one owner to bear burdens that should be borne by the public.
Yes. As applied to Pennsylvania Coal's reserved mineral estate and the express waiver of surface support, the Kohler Act went too far and effected a taking, violating the Takings Clause. The injunction enforcing the statute could not be sustained.
Mahon is the cornerstone of regulatory takings law. It introduced the idea that regulations can be takings and identified guideposts—extent of diminution, character of the action, reciprocity of advantage—that later matured into the Penn Central balancing test. It also sparked enduring debates: how to define the relevant parcel (Holmes's analysis effectively severed the support estate, later constrained by the "parcel as a whole" approach), how to treat harm-preventing regulations (Brandeis's view resurfaces in Keystone), and when compensation is due for severe value loss (refined in Lucas for total economic wipeouts). For law students, Mahon frames the analytical vocabulary and tensions every takings exam problem invokes.