Three kilometres under the East Antarctic plateau sits air that nobody has ever breathed. It fell as snow around 800,000 years ago, was buried by the following winter, and then by every winter since.

Drillers with the European Project for Ice Coring in Antarctica spent close to a decade going after it at a site called Dome C, stopping a few metres short of bedrock. What came up was a stacked record of eight ice ages and eight warm periods, in the order they happened. An editorial in Nature Geoscience marking the result put the timescale in human terms: the deepest ice holds the air that Homo antecessor breathed, a hominin species that long preceded the Neanderthals.

How a snowfall becomes a sealed sample

Fresh Antarctic snow is a loose heap of crystals with air moving through the gaps, and it keeps swapping that air with the atmosphere for decades. Each new snowfall presses down on the last, the heap compacts into a layer called firn, and the passages between grains eventually pinch shut. Michael Bender, Todd Sowers and Edward Brook set out that sequence in a PNAS review of gases in ice cores. The close-off depth, usually within the first hundred metres, is where a parcel of atmosphere stops being weather and becomes a specimen.

One consequence is that ice and the air inside it keep slightly different time, because the gas is still moving around for a while after the ice holding it has stopped. At Dome C, where very little snow falls in a year, that gap runs into thousands of years. Every dating model has to carry a correction for it.

No figure that comes out of this ice arrives without an error bar attached.

What happens to the bubbles further down

Squeeze a bubble hard enough and it stops behaving like a bubble. Under a kilometre or more of overlying ice, trapped air gets absorbed into cages in the crystal structure itself, forming what are called air clathrate hydrates. Bender and his co-authors put that conversion at roughly 400 to 1,500 metres, depending on how cold and how dry the site is. Ikumi Oyabu and colleagues at Japan’s National Institute of Polar Research, writing in The Cryosphere, mapped the zone at Dome Fuji. Bubbly ice runs down to about 450 metres, a transition zone from there to 1,200 metres, and below that, no bubbles at all.

So the oldest air in a deep core is not sitting in bubbles. It has been rehoused molecule by molecule into the ice lattice, where it stays just as sealed and just as measurable.

What the ancient air said

Carbon dioxide and Antarctic temperature rise and fall together across all eight cycles, which is the finding that made the core famous. Across the ice age cycles of the past million years, per NOAA’s account of the ice core record, carbon dioxide never climbed above 300 parts per million.

Then there were the two dips that did not fit. The same Nature Geoscience editorial flagged them. Twice between 800,000 and 650,000 years ago, carbon dioxide dropped below the 180 to 280 ppm envelope that had been treated as the natural range, while Antarctic temperatures sat at ordinary glacial values. One of those excursions reached 172 ppm, the lowest level ever measured in ancient ice.

For scale, Scripps Institution of Oceanography calculated a monthly average of 430.2 ppm at Mauna Loa in May 2025, with NOAA’s own laboratory putting the figure at 430.5.

The 800,000-year record runs along inside a narrow band, then goes vertical at one end.

Getting the gas out without wrecking it

Extraction means crushing or melting the ice in a vacuum and feeding what comes off into a gas analyser, which is where the paranoia shows. Dieter Lüthi and co-authors, reporting in Nature, worked from the lowest 200 metres of the Dome C core, which bottomed out at 3,260 metres. Measurements were split between the University of Bern and a laboratory in Grenoble, with alternating slices analysed independently at each, so no single instrument could quietly set the record on its own.

That caution earned its keep in 2015. Bernhard Bereiter and colleagues reported in Geophysical Research Letters that one of the extraction systems had introduced a bias of around 10 ppm in exactly that deepest stretch. They traced it to incomplete gas release, combined with the way stored ice relaxes over years in a freezer. Their corrected record went some way towards resolving the odd temperature relationship at the bottom of the core.

Where the record goes next

A newer camp at Little Dome C, about 35 kilometres from the old one, has already gone further back. The British Antarctic Survey announced in January 2025 that the Beyond EPICA team had reached bedrock at 2,800 metres and recovered ice more than 1.2 million years old. That covers a stretch when the rhythm of the ice ages changed, for reasons still argued over.

All of it rests on an awkward arrangement. This record of the climate is made of ice, and ice is the first thing a warm spell destroys. Every interglacial in those 800,000 years, including the warm periods documented in the core itself, stayed cool enough at Dome C to leave the layers stacked and legible. If one of them had run hotter, the evidence of it would have melted along with the evidence of everything underneath.