Lost leaks are leaks that disappear from utility-reported lists of unrepaired leaks with a record of being repaired. We analyzed the fraction of leaks lost by the three major utilities in Massachusetts for the past three years. The fraction of lost leaks year to year ranges from 6% to 21%. While the vast majority of lost leaks do not pose an explosion risk, they result in emissions of methane, a potent greenhouse gas. Our lost leak results are conservative because we only count leaks as lost if there a no repairs within 200 feet of the leak location, and some of these repairs may be addressing other nearby leaks. Use the layers box in the upper right corner to display repaired, unrepaired, and lost leaks for different utilities and years. This work is conducted in collaboration with HEET and the Gas Leaks Allies.