For Lars Peter Nielsen, it all began with the mysterious disappearance of hydrogen sulfide. The microbiologist had collected black, stinky mud from the bottom of Aarhus Harbor in Denmark, dropped it into big glass beakers, and inserted custom microsensors that detected changes in the mud’s chemistry. At the start of the experiment, the muck was saturated with hydrogen sulfide—the source of the sediment’s stink and color. But 30 days later, one band of mud had become paler, suggesting some hydrogen sulphide had gone missing. Eventually, the microsensors indicated that all of the compound had disappeared. Given what scientists knew about the biogeochemistry of mud, recalls Nielsen, who works at Aarhus University, “This didn’t make sense at all.”