Ice sheets are large and complex things. Figuring out how quickly—and where—they’ll melt as the world warms is a monumental task. We worry about some portions (like the vulnerable West Antarctic Ice Sheet) collapsing entirely, but we know some other parts will be disappearing in the foreseeable future. Records from past periods of climate change are important guides here. What better way to figure out what will happen than to see what has happened before?

For the Greenland Ice Sheet, there has been some debate about how small it has gotten in past warm periods where we know sea level was higher than it is today. The problem is that Greenland’s ice doesn’t go nearly so far back in time as Antarctica’s. Snowfall is greater here, and ice flows more quickly to the edges of the continent where it disappears from the pages of the history we read from ice cores. Few Greenland cores go back more than about 110,000 years, failing to tell us about the last interglacial warm period.

But at the bottom of a couple of ice cores from the thickest parts of the ice sheet, there is some messed up ice we know could be a lot older. Figuring out how old is another matter. Without an orderly stack of annual layers to count back through, there aren’t many reference points preserved in the ice. To make things worse, water can refreeze to the underside of glaciers, so it might not have even been glacial ice in the first place.