Coal mining companies that were primarily interested in the coal beneath the land’s surface often sold the land above the mines – but only the surface rights. Sometimes, the surface buyers would specify in their contracts with the mining companies that special supports had to be installed in the mines when the coal was removed to prevent cave-ins – but not always, because it cost less to buy the surface rights without requiring the companies to guarantee support. Over the years, towns were built above coal mines. In 1921, in response to a growing problem of cave-ins above unsupported mines, the state of Pennsylvania passed the Kohler Act to protect the public from these accidents. In this case, surface owners sued under the Kohler Act, asking that all mining beneath their property be stopped. In Pennsylvania Coal Co. v. Mahon, the U.S. Supreme Court rules that the Kohler Act violates the takings clause of the Fifth Amendment. The Court finds that the surface owners knew that they had only limited rights to the property when they bought the land and that they must absorb the risk and the cost of any repairs, unless the contract specifies that the coal company is liable for damages.