So, to declare an Age of Humans, scientists would need to be convinced of a human impact visible within Earth’s rocks layers. And for some years, scientists have proposed that we should be able to find that impact in the form of a layer of plutonium laid down in rock after nuclear weapons testing in the 1950s. Scientists call that a golden spike – evidence in Earth’s sediment layers – clearly demarcating one geologic epoch from another.

Out of many possible sites, geologists picked Crawford Lake in Ontario to look for the golden spike. Specifically, they chose the lake because of the clear annual deposits in its sediments. It’s a small body of water, just about half the size of a football field. But also, it’s about 80 stories deep. It lies southwest of Toronto and west of giant Lake Ontario.

And now, says the Anthropocene working group, the golden spike – the telltale plutonium layer – has been found in the lake.