Pennsylvania Coal Co. v. Mahon

When Pennsylvania Coal Co. (defendant) sold the surface of a parcel of land in 1878, it expressly retained the right to mine coal beneath the property, and the deed stated the buyer took the land subject to the risks of subsurface mining; the buyer's successor, Mahon (plaintiff), later sued to stop Pennsylvania Coal from mining after Pennsylvania enacted a 1921 statute barring mining that could compromise the integrity of any surface land. The trial court found mining would damage the surface but declined to enjoin it, finding the statute unconstitutional, while the Pennsylvania Supreme Court reversed, upholding the statute as a valid police-power exercise and ruling for Mahon; the case reached the U.S. Supreme Court by writ of error.