Nuclear weapons testing in the 1950s and early 1960s left the first obvious and indelible marks of "overwhelming" human activity on Earth, and these events may signal the beginning of a new geological epoch known as the Anthropocene, geologists say.
Fallout from these tests snowed down from the atmosphere and became trapped in the earth as layers of sediment rich in a radioactive form of plutonium, called plutonium-239.
