Three kilometres beneath the East Antarctic plateau resides air that nobody has ever inhaled. It precipitated as snow approximately 800,000 years ago, was hidden by the subsequent winter, and has been enveloped by each winter since then.
Drillers associated with the European Project for Ice Coring in Antarctica dedicated nearly a decade to pursuing it at a location named Dome C, ceasing just a few metres short of bedrock. What they retrieved was a consolidated record of eight ice ages along with eight warm periods, presented in the sequence in which they occurred. An editorial in Nature Geoscience commemorating this outcome contextualized the timeline in terms relatable to humans: the oldest ice contains the air once inhaled by Homo antecessor, a hominin lineage that existed long before the Neanderthals.
## The journey from snowfall to sealed sample
Freshly fallen Antarctic snow appears as a loose collection of crystals with air circulating through the spaces, constantly exchanging that air with the atmosphere for decades. Each new snowfall compacts the previous layer into a structure known as firn, eventually closing off the gaps between the grains. Michael Bender, Todd Sowers, and Edward Brook outlined this progression in a PNAS review focused on gases in ice cores. The depth at which this closure occurs, typically within the initial hundred metres, is where a volume of atmosphere ceases to be weather and transforms into a specimen.
One implication of this process is that ice and the air contained within it maintain slightly varying timeframes, as the gas continues to move for some time after the ice encasing it has solidified. At Dome C, where annual snowfall is minimal, this discrepancy spans thousands of years. Any dating model has to incorporate a correction factor for it.
No data extracted from this ice emerges devoid of an attached error margin.
## The fate of bubbles deeper down
If a bubble is compressed sufficiently, it ceases to function as a bubble. Beneath a kilometre or more of overlying ice, trapped air gets absorbed into cages within the crystal structure, creating what are termed air clathrate hydrates. Bender and his co-authors estimated this transformation happens at approximately 400 to 1,500 metres, varying based on local temperature and dryness. Ikumi Oyabu and colleagues at Japan’s National Institute of Polar Research, in a publication within The Cryosphere, mapped the zone at Dome Fuji. Bubbly ice persists down to about 450 metres, transitions from that level down to 1,200 metres, and below that depth, no bubbles are found.
Thus, the oldest air in a deep core does not remain in bubbles. It has been sequentially housed molecule by molecule within the ice lattice, where it remains securely contained and measurable.
## Insights from the ancient air
Variations in carbon dioxide and Antarctic temperature rise and fall synchronously across all eight cycles, which is the discovery that brought fame to the core. Throughout the ice age cycles of the past million years, according to NOAA’s account of the ice core data, carbon dioxide levels never exceeded 300 parts per million.
However, two anomalies did not conform. The same Nature Geoscience editorial highlighted them. Twice between 800,000 and 650,000 years ago, carbon dioxide levels dropped below the 180 to 280 ppm envelope that had been regarded as the natural range, while Antarctic temperatures remained at typical glacial levels. One of those anomalies reached 172 ppm, the lowest concentration ever recorded in ancient ice.
For context, the Scripps Institution of Oceanography calculated a monthly average of 430.2 ppm at Mauna Loa in May 2025, with NOAA’s own laboratory reporting the figure at 430.5.
The 800,000-year record traverses a narrow band before rising sharply at one end.
## Extracting gas without causing damage
Extraction involves crushing or melting the ice in a vacuum and directing what is released into a gas analyser, where the cautious approach becomes evident. Dieter Lüthi and co-authors, reporting in Nature, worked with the lowest 200 metres of the Dome C core, which extended down to 3,260 metres. Measurements were divided between the University of Bern and a laboratory in Grenoble, with alternating samples independently analysed at each location, ensuring no single instrument could quietly determine the record on its own.
This carefulness proved valuable in 2015. Bernhard Bereiter and colleagues reported in Geophysical Research Letters that one extraction system had induced a bias of about 10 ppm in precisely that deepest section. They traced it back to inadequate gas release, combined with the behavior of stored ice over the years in a freezer. Their adjusted record contributed to clarifying the peculiar temperature relationship at the core’s bottom.
## The future of the record
A new site at Little Dome C, located roughly 35 kilometres from the previous one, has already delved further back in time. The British Antarctic Survey announced in January 2025 that the Beyond EPICA team reached bedrock at 2,800 metres and retrieved ice that is over 1.2 million years old.