Back in 2010, a group of scientists drilling in Antarctica pulled up a one-million-year-old chunk of ice. At the time, it was the oldest ice core ever discovered. But as Paul Voosen reports for Science, the team recently dug even deeper into Earth’s glacial history, unearthing an ice core that dates back 2.7 million years.

The chilly discovery was made in the Allan Hills region of Antarctica, in an area of largely untouched blue ice. Typically, as Sarah Laskow explains in Atlas Obscura, scientists drill into ice made up of continuous layers, each one compacted over time. But that type of ice does not preserve its oldest layers, which eventually are melted by the Earth’s internal heat. The team consequently looked to blue ice, which is layered not by age, but rather is formed in exposed areas where any net addition or subtraction of snow is mitigated due to wind and sublimation. It is because of that, Voosen writes, that “old layers are driven up...revealing the lustrous blue of compressed ice below."

There is a drawback to studying blue ice, however; because it is not organized into neat layers, it is difficult to date. So Michael Bender, a Princeton geochemist, devised a solution that involved measuring the amount of argon and potassium contained within a piece of ice. It isn’t the most accurate method—there is a margin of error of about 100,000 years—but it can give researchers a fairly good picture of an ice core’s age.

But why, you may ask, are researchers on the hunt for ancient ice? As Trevor Nace explains in Forbes, ice cores from the Arctic and Antarctica can tell us a lot about the climates and atmospheres of past epochs. When snow first falls, it is fluffy and airy; over time, as it get covered with successive layers of snow, it becomes compacted, its air pockets are forced out and it begins to transform into ice. But even ancient ice contains tiny bubbles—and those little bubbles have roughly the same air composition as they did when the original layer of snow first fell.

The team’s findings, which were presented at the Goldschmidt Conference in Paris, revealed that the ice dating back 2.7 million years contained air bubbles that did not exceed 300 parts per million (PPM) carbon dioxide—in comparison to the levels of carbon dioxide in the air today (which exceeded 410 PPM for the first time in millenia this April). The ice may offer be a from the beginning of an ice age; as Laskow points out, experts have theorized that such low carbon dioxide levels played a role in pushing Earth into a series of significant cold periods.

Moving forward, the team plans to continue exploring blue ice, in search of ice dating back five million years. According to Nace, they are looking to go back to a time when carbon dioxide levels were comparable to what they are today. By unearthing Earth’s frosty history, they hope to be able to better understand where the planet is heading in the future.