In 2016, ornithologist Steven Emslie was studying penguin colonies along the shores of Antarctica’s Ross Sea, when he heard about pebble mounds associated with guano deposits on Cape Irizar, a rocky outcrop surrounded by water and ice.

Guano forms as the corrosive poop of generations of nesting birds reacts with rocks and soil. But no active colonies have ever been recorded at the site, even by early Antarctic explorers spotting and naming the cape in 1901.

Visiting the site, Emslie came across of what seemed at first a fresh penguin carcass resting on older remains, including bones, feathers, and eggshell. Later sampling and carbon-dating put the carcass at a minimum of 800 to 1,100 years old, with some of the older remains found at the site dating back to 1,375-2,340 and 2,750- 5,145 years ago. The pebble mounds were nests, suggesting that the site was a former penguin colony.

The varying ages imply at least three periods of occupation and abandonment of this site, according to the paper published in the journal Geology.

The Southern Ocean surrounding Antarctica supports nearly one million breeding pairs of Adélie penguins (Pygoscelis adeliae) annually. There also is a well-preserved record of abandoned penguin colonies on the continent that date from before the Last Glacial Maximum some 45,000 years ago.

As penguins need pebbles for their nests, during breeding season they will visit ice-free spots. Emslie speculates that the penguins abandoned the newly discovered colony during the onset of the Little Ice Age, a period of cooling between 1300 to 1850, which covered the former ice-free area in snow and ice. With the average annual temperature of this part of the continent rising nowadays by between 1.5 and 2 degrees Celsius since the 1980s, the resulting snowmelt exposes the remains, and decomposition begins.