For thousands of years, several colonies of Adélie penguins lived in secret on Antarctica’s Danger Islands. Researchers recently found the groups of small, flightless birds—more than 1.5 million of them—by analyzing images taken from a NASA satellite. What gave the penguins away? The telltale pink color of the birds’ poop, called guano, which stood out against the gray rock and white snow of the Antarctic.

“When we first spotted the penguins from space, we thought it was a mistake,” says Heather Lynch, a biologist at Stony Brook University in New York. She and a team of scientists traveled 11,500 kilometers (7,100 miles) to the Danger Islands to take a look for themselves. The expedition confirmed the existence of several massive colonies of Adélie penguins.

“We were definitely not expecting to find penguins here,” Lynch says. But now that they have, the islands are being added to a proposed protected area to help preserve the newly discovered birds.