In 1878, Pennsylvania Coal Co. (Pennsylvania Coal) (defendant) conveyed the surface of a plot of land it owned to Mahon (plaintiff). In this transfer, Pennsylvania Coal retained the right to mine underneath the property, and an explicit provision in the deed stated that Mahon was taking the land subject to any risks associated with mining beneath the land. In 1921, the Commonwealth of Pennsylvania enacted a statute preventing coal mining that could possibly affect the integrity of any surface land. Mahon then sued Pennsylvania Coal, arguing that the new state law barred Pennsylvania Coal from mining under the property. The trial court concluded that Pennsylvania Coal would damage the surface of the land if it kept mining, but declined to issue an injunction on the ground that the statute was unconstitutional. Mahon appealed to the Pennsylvania Supreme Court, which concluded that the statute was a valid exercise of the state's police powers and ordered judgment be entered in Mahon's favor. The Pennsylvania Supreme Court then sent the case to the United States Supreme Court by writ of error, which was granted.