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Imagine the most profound Ice Age you can think of. Temperatures plummet to below zero even at the equator. Before long, ice encloses our entire planet. Research suggests that this may have happened not once, but three times in our planetfs past. Welcome to snowball Earth. A snowball Earth is certainly the most extreme climate change we can conceive the Earth having ever experienced. What would happen if the world froze over today? Massive glaciers creep from the poles, crushing everything in their path. First of all, the Canadians are gonna have to dig much deeper holes when they go ice fishing. Next, the governors of Washington, Idaho, Montana, North Dakota, Minnesota, Michigan, and Maine will probably declare federal emergencies. Cities grind to a halt. Power lines collapse and water supplies freeze. Cars wonft start. Roads are cut off by ice. Buildings strain and crumble under deep drifts of snow. Everything is entombed. Our frail bodies struggle to survive. I think human society would collapse if a snowball Earth happened again. The oceans freeze over. Fish die and marine ecosystems collapse. Our food supply becomes exhausted. Any surviving humans are trapped in an icy hell. Whatever mass die-off of humans is simply because we could not grow enough food under these conditions. Incredible though it may seem, the Earth may have frozen over completely. Scientists are finding clues in unexpected places. The story begins with a mystery. In the arid Flinders Ranges in Southern Australia, layers of ancient rock hold unusual stones. These stones are foreign to the area. They are known as drop stones. The drop stones are embedded in an ancient sedimentary sea floor that formed around 650 million years ago. Tim Raub, a geologist at the Californian Institute of Technology, hunts for these drop stones at the Flinders Range. This is an example of a drop stone. A piece of ancient rock that was dropped into the ancient mud on the sea bottom. These drop stones are visibly different from the sedimentary bedrock around them. Something powerful must have carried them here. But what? Dan Schrag, Professor of Earth and Planetary Sciences at Harvard University, believes the answer is glaciers. We actually have ice cubes here that wefve made with pieces of sand and gravel in them...so this - these ice cubes are like the glaciers that are taking rocks from the land, and then when we put them in the water, theyfll actually melt...and the sand and gravel will fall to the bottom, just like the drop stones on the ocean floor. These drop stones are evidence that at some point, this area was covered in a thick layer of ice.