Blood Falls is an aptly named feature in Antarctica. The 100-foot stream of water running down the side of a glacier is a deep, rich, blood red.

Though we’ve known for decades what causes the red color, it took more than 100 years for scientists to discover the source of Blood Falls: a secret, ancient, underground lake.

Blood Falls were first discovered by Australian explorer Griffith Taylor during an expedition in 1911. At the time, he and other explorers guessed that the red color might be caused by algae living in the water.

“It's unearthly, it's unreal,” Steve Martin, an Antarctic historian, told Motherboard on the latest episode of Science Solved It. “So when [explorer] Griffith Taylor and his friends saw the blood falls flowing red out of the end of the Taylor Glacier, they must have thought it was just another incredible oddity in a very strange part of the world.”

Though scientists later realized it was the high iron content that turned the water bloody red (the water oxidizes and turns red when exposed to air), they still didn’t know where the water was coming from or how the falls formed.

“We did not know where the brine came from. We didn't know how it made its way through the glacier,” explained Erin Pettit, one of the scientists who solved the mystery of Blood Falls. “If [the brine] started at the base of the glacier, it should have continued to flow at the base.”

Instead, the brine squirted out the top of the glacier and flowed down over the edge, eventually joining a nearby lake.

To figure out how it formed, Pettit and her team trekked across the glacier and took measurements using a radio-wave sensor. The instrument sends radio pulses into the ice, which move freely through the frozen glacier.

However, when the waves hit the salty, slushy water flowing to Blood Falls, they scatter, allowing the researchers to map exactly where the water was snaking through the glacier. They eventually identified that the immense pressure of the ice squeezed the water trapped in an ancient, underground lake below up through the glacier.

“It was pretty powerful, pressurize brine in that conduit,” Pettit said. “Even though it's not always squirting out the top of the glacier, it's always sitting within the ice there as a pressurized, slushy ice mess.”

The team published their findings and were able to confirm them when a drilling team visited the region the following year. By using the map Pettit and her team had created, the drilling crew located where the underground source should be and got to work. Sure enough, red brine squirted up around the drill.

Along with finding the source of Blood Falls, the scientists also provided more context for life forms that had previously been discovered there: tiny microbes capable of surviving in super salty, high-iron, very cold water, without sunlight, under a glacier. It turns out these extremophiles were even more extreme than previously recognized, and studying them further can help us understand how life might survive in other extreme environments, such as outer space.

“That first discovery is leading us on a track of further discovery and further explanation,” Martin said. “Antarctica still has not yielded up all her secrets.”