A few weeks back, the dramatic rescue of an ailing scientist from the South Pole was front-page news, and focused attention on the research that's going on year-round at the bottom of the world. Much of that work involves what's above Antarctica in the atmosphere - for example, the ozone hole. But a lot of the research has to do with what lies below - the ice itself.

Antarctica is really just a big slab of ice and snow resting on land. In places, the ice is more than a mile thick, and it's taken hundreds of thousands of years to accumulate. The deeper you drill into the ice, the farther back in time you go. And so studying ice cores taken from the Antarctic ice sheet is a bit like reading a history book.

One of the most revealing clues comes from tiny bubbles of air that get trapped in the ice when it forms. Essentially, these bubbles are samples of ancient air. One important result derived from these pockets of trapped air is about carbon dioxide, the primary gas implicated in global warming: we've learned that carbon dioxide levels do vary naturally over time, but the current concentrations are higher than at any time in at least the last 200,000 years, and probably longer than that.