In 1966, the Pennsylvania legislature enacted the Subsidence and Land Conservation Act (Subsidence Act), which prohibited mining coal when doing so might cause subsidence of land. Land subsidence due to coal mining is a significant problem in Pennsylvania that causes widespread property damage. The Keystone Bituminous Coal Association (Keystone) (plaintiff) brought suit in federal court against the various state officials (defendants) in charge of determining what coal could not be mined and of enforcing the prohibition on mining. Keystone claimed that the Subsidence Act’s prohibition on mining coal constituted a taking requiring just compensation. Keystone also claimed that the regulations were a taking because they prevented the mining companies from exercising their rights to the support estates of their land (Pennsylvania recognizes three types of estates: surface estate, mineral estate, and support estate). At the time the suit was brought, the law had not been used to prevent any coal from being mined. The trial court ruled that there had not been a taking, and the circuit court affirmed. Keystone appealed to the Supreme Court.