It's a well-established scientific fact that humans have had a massive impact on the planet. But has it been big enough to warrant the definition of a new geological epoch? It's an idea that's been hotly debated in the scientific community for years — and now, a group of researchers are arguing that a small lake in rural Ontario provides the best evidence for defining that new epoch. Crawford Lake, about 60 km southwest of Toronto, captures the history of the world in its sediment deposits, calcified like tree rings. Scientists say those layers show dramatic changes starting in the 1950s and that they mark a new geological epoch called the Anthropocene. Canadian Geographic contributing editor Alanna Mitchell explains the latest research, what makes Crawford Lake so special, and why defining the Anthropocene has been causing scientific controversy for more than two decades. For transcripts of this series, please visit: https://www.cbc.ca/radio/frontburner/transcripts