Scientists have provided perhaps the first direct evidence that a comet struck Earth around the time of the Paleocene-Eocene Thermal Maximum, a notable warming event more than 50 million years ago, and the cause of which has been a topic of debate. In recent years, the Paleocene-Eocene Thermal Maximum, or PETM, has become a major point of interest for scientists as it is perhaps the best past analog by which to understand impacts of global climate warming today. Even so, the cause of this warming event remains a mystery. Among the suggested drivers of the observed massive injection of carbon into the atmosphere that occurred at this time are the intrusion of flood basalts into carbon-rich marine sediments, carbon degassing from volcanos, and an extraterrestrial impact on Earth. Here, by studying sections of marine shelf on the Atlantic Coastal Plain associated with the Paleocene-Eocene boundary, Morgan Schaller and colleagues report the discovery of silicate glass spherules with distinctive morphologies and micro-craters; based on their features, the authors say, these glass spherules are best interpreted as terrestrial debris ejected during a meteorite impact. The results suggest that a comet impact occurred near the Paleocene-Eocene boundary, although it still is unclear exactly when this impact event occurred in relation to the PETM or what effect it may have had on this period of global environmental change.