Humans have so profoundly altered the Earth that, some scientists argue, our current geologic epoch requires a new name: the Anthropocene. But defining the precise start of the era is tricky. Would it begin with the spread of domesticated farm animals or the appearance of radioactive elements from nuclear bomb tests? Scientists report in ACS’ Environmental Science & Technology a method to measure levels of human-made contaminants in sediments that could help pinpoint the Anthropocene’s onset.