Each year's snowfall in Antarctica settles in a distinct layer, and as the layers compress into ice they trap tiny bubbles of the atmosphere as it was at the time. Drilling down through the ice sheet is therefore a form of time travel: an ice core three kilometres long can contain a continuous record of the air stretching back eight hundred thousand years. From these bubbles scientists can read past concentrations of carbon dioxide directly — not infer them from indirect evidence — which is why ice cores, rather than tree rings or ocean sediments, anchor our understanding of how today's atmosphere compares with the deep past.