Scientists detailed their discovery of the new super-colony in a new paper published this week in the journal Scientific Reports.
"It's a classic case of finding something where no-one really looked!"
Scientists from Oxford University were part of an worldwide team searching for images taken by the Landsat satellites to locate possible large numbers of penguin by identifying the mess they leave behind.
Landsat does not return especially high-resolution pictures and so when the system flagged potential colonies, they had to be followed up with much sharper pictures for confirmation.
"Is certainly surprising and it has real consequences for how we manage this region", explained Heather Lynch, the study's author and a researcher at the Stony Brook University. It's locked up in sea ice most of the year, and even in summer it's hard to reach.
This has allowed the penguins to remain hidden from the world, until a team of researchers mounted an expedition there to investigate signs of nesting birds.More news: Rachel McAdams is pregnant
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"Not only do the Danger Islands hold the largest population of Adélie penguins on the Antarctic Peninsula, they also appear to have not suffered the population declines found along the western side of Antarctic Peninsula that are associated with recent climate change", said Professor Michael Polito, an ecologist at the Louisiana State University who participated in the research.
Scientists had theorised the decline was linked to reductions in sea-ice, which is an important habitat for krill, the small crustaceans that form a key part of the penguin diet.
There are more than 750,000 pairs of penguins living on the islands - 1.5 million animals in total - which is more than the rest of the Antarctic Peninsula combined.
"It's not clear what the driver of those declines is yet; the candidates are climate change, fishing and direct human disturbance, but it does show the size of the problem". "Finally getting into the Danger Islands and counting the penguins shows how robust populations are where the ice is intact".
"We want to understand why".
Dr Peter Fretwell from the British Antarctic Survey knows the team but was not involved in this study. It's also the middle of the breeding season for penguins, which helped in counting them.
Pygoscelis adeliae is commonly known as the Adélie penguin, after the wife of French explorer Jules Dumont d'Urville, the man who first documented them in 1840. "I am sure that there are many other natural discoveries to be made using these "eyes in the sky".