There aren’t too many places left on Earth where one can hide completely undetected, but that’s what a “mega-colony” of penguins has been doing on a remote string of islands off Antarctica.

Scientists were recently stunned to discover that more than 1,500,000 Adelie penguins have been living in obscurity on the Danger Islands off the Antarctic Peninsula's northern tip.

The penguins managed to go undetected for so long because the islands where the birds live are so difficult to access that, even in the summer, ice-filled waters surrounding the islands make human visits too treacherous.

Dr. Tom Hart of Oxford University’s department of zoology who was part of the team that found the penguins, told The Daily Telegraph it’s amazing that so many birds could go undetected for so long.

“The weirdest, most surprising and incredible thing is that, in this day and age, something so big can go unseen,” he said.

In a new paper in the journal Scientific Reports, scientists with the Woods Hole Oceanographic Institution (WHOI)  explain how they managed to spot the birds after noticing unusually large patches of excrement in images of the islands taken by NASA satellites, in 2014.

The team launched an expedition to the islands in December, 2015. They sent up a drone to take aerial images of the islands and then used a Deep Neural Network to estimate the number of birds.

Hanumant Singh, professor of Mechanical and Industrial Engineering at Northeastern University, helped develop the drone's imaging system and explained it could take a picture once per second as it flew in a grid pattern.

“You can then stitch (the images) together into a huge collage that shows the entire landmass in 2D and 3D," he said in a statement.

That collage revealed thousands of nests where 751,527 pairs of Adelie penguins were living – more than the rest of the entire Antarctic Peninsula region combined.

Adelie penguins are the smallest species of penguin in the Antarctic, weighing just 3 to 6 kg. They live all over the continent, but in recent years, their populations have been dwindling due to climate change.

Between 2010 and 2017, for example, 18,000 Adelie chicks on the other side of Antarctica died of mass starvation after thick ice made feeding too difficult.

But the WHOI says the super-colony discovered on the Danger Islands seems to be doing rather well, with a population that has likely been stable for decades.

“Not only do the Danger Islands hold the largest population of Adelie penguins on the Antarctic peninsula, they also appear to have not suffered the population declines found along the western side of Antarctic peninsula,” study co-author Michael Polito, from Louisiana State University’s department of oceanography, said in a statement.

Study co-author Stephanie Jenouvrier, a seabird ecologist at WHOI, says it’s important to now understand why the population of Adélies on the islands is so different from the west side of the Antarctic Peninsula.

“We want to understand why. Is it linked to the extended sea ice condition over there? Food availability? That's something we don't know," she said.