Roughly 15 years ago, many Pennsylvania landowners entered into oil and gas leases in the expectation of receiving royalties for the extraction of shale gas and oil underlying their properties. Today, as natural gas prices remain low, and fossil fuels generally—including methane leaks specifically—encounter policy and regulatory resistance because of attention to climate change, lessees may not move as rapidly to develop marginal leaseholds. Indeed, some may not continue exploration or production at all, raising the question of when a lease ends.  Last month, the Pennsylvania Supreme Court offered some guidance in SLT Holdings v. Mitch-Well Energy, No. 6 WAP 2020 (Pa. Apr. 29, 2021).