>> nikita zimov: yeah. >> pelley: nikita zimov grew up here with his father, and sensibly moved souththe old man and the river. but nikita's plan to be a mathematician melted away when sergey asked his son to return, to see what he had seen. a few hours from the research station, there's a vast subsidence of permafrost-- sort of a rolling landslide, called duvanny yar. geology is a slow science, but here, it's almost a spectator sport. the bones of extinct woolly mammoths are thawing, after more than 12,000 years. the collapse of frozen earth is happening in much of the arctic, including alaska. 25% of the northern hemisphere is permafrost. the zimovs have a theory-- many would say a crazy idea-- for defusing the carbon bomb. they want to cool the permafrost by returning part of siberia to the ice age, or at least what it looked like in those days, known as the pleistocene era. if we were standing on this hill in the pleistocene era, what would we see? >> sergey zimov: not any trees. this looks like grasslands and savanna. and you will see around 1,000 of mammoths, around maybe 5,000