The world is a frozen ball. From the poles to the equator there is nothing but ice. The mountains are covered by snow, the valleys are filled by glaciers and across the plains, ice sheets are kilometres thick. From the poles to the equator there is no water. Ice covers the oceans, barely breaking into floes across the tropics. It never rains. Where the sun is hottest a little water vapour sublimes from the ice, eventually to fall as snow. The whole Earth is silent.

But not still. Beneath the ocean, erupting volcanoes pour lava onto the sea floor and gases into the cold water. The gases remain there, dissolved; kept in by the blanket of ice. On land, too, volcanoes erupt. Out of the rifts pours carbon dioxide as well as molten rock. Like the gases in the ocean, the CO2 in the atmosphere stays there, for it has nowhere to go. There are no plants to take it up, no open ocean to absorb the CO2, and no rocks are exposed to weathering. Slowly, the levels of CO2 in the air rise, and slowly the world warms, until at last some of the sea-ice melts. Now, the CO2 in the oceans also starts to escape into the air, and as it does the opening ocean absorbs ever more of the Sun’s heat. Together, the increased CO2 and the darker surface of the seawater combine in a feedback loop that, after perhaps 10 million years, releases the icy grip of ‘snowball’ Earth.