He walks through the sheer scale of the Late Glacial Maximum, when at least six million cubic miles of ice locked up the Northern Hemisphere. To visualize that, he asks you to picture a single cubic mile of ice, a block nearly three times the height of One World Trade Center. Now multiply that by six million. The numbers stop being abstract when you realize what happens if that ice melts rapidly. You are not talking about gradual sea level rise over centuries. You are talking about catastrophic flooding on a scale that rewrites coastlines and erases entire landscapes. The evidence for such events is not theoretical. It is carved into the geology of North America.