A strange and disturbing sight way up in Antartica has baffled scientists since it was first discovered in 1911. Geoscientist Griffith Taylor stumbled upon what he would later call “Blood Falls” in 1911, so named because of a red river that spilled over a cliff on the continent with an unexplained origin, but scientists now say they know what’s causing it.

Obviously, it’s not actually blood, but an iron-rich brine that oxidizes when coming in contact with the air, not algae as scientists had first theorized when they learned of the phenomenon. The red flow is basically liquid rust, based on a study by scientists at the University of Alaska Fairbanks.

As for where it comes from, scientists were able to use special equipment to discover a small saltwater lake trapped beneath the glacier that may be a million years old. The lake is so incredibly salt that it doesn’t freeze, and it scrapes up iron out of the bedrock, which eventually seeps through the ice and oxidizes, creating the bizarre red color.

“The salts in the brine made this discovery possible by amplifying contrast with the fresh glacier ice,” Lead author Jessica Badgeley said.

“We moved the antennae around the glacier in grid-like patterns so that we could ‘see’ what was underneath us inside the ice, kind of like a bat uses echolocation to ‘see’ things around it,” said co-author Christina Carr, a doctoral student at UAF.

“While it sounds counterintuitive, water releases heat as it freezes, and that heat warms the surrounding colder ice,” University of Alaska Fairbanks glaciologist Erin Pettit said. “Taylor Glacier is now the coldest known glacier to have persistently flowing water.”