Greenland's ice sheet is three kilometers thick, built up over millions of years, and so heavy it has pushed the island below sea level. This installment of the Cosmic Journeys series asks what happens if it goes: sea levels rising more than seven meters, enough to threaten homes for roughly a quarter of the world's population. To find out where that threat is heading, scientists drill into ice cores, scour ancient seabeds, fly radar surveys over polar ice, and track it all by satellite, reconstructing a climate record that stretches back over two billion years through five major ice ages, from the Huronian to the current Quaternary. Computer animation reconstructs the slow drift of continents, the rise of the Himalayas, and the spread of an Arctic fern called Azolla that once pulled tons of carbon dioxide from the air. Milutin Milankovic's nineteenth-century work on Earth's orbital wobble gets its due too, explaining the roughly 100,000-year rhythm of freezing and thaw the planet has followed long before humans entered the picture.