Scientists think they have finally cracked the code of one of the most befuddling mysteries on the continent of Antarctica. They think they’ve determined what is causing the blood-like substance that pours into the sea off a cliff that was first discovered in 1911 and was subsequently named “Blood Falls.”

Not surprisingly, it’s not really blood, but rather an iron-rich brine that oxidizes when it comes i contact with the air, and not an algae in the water that experts had assumed in the past. Basically, it’s rust in water form, scientists at the University of Alaska Fairbanks discovered.

It apparently comes from a small saltwater lake that is trapped beneath a glacier and has been there for a very long time, perhaps a million years. The lake can’t freeze bcause it is so salty, and so it scrapes up iron from the bedrock as it seeps through the ice, creating the strange phenomenon.

“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. The heat and the lower freezing temperature of salty water make liquid movement possible. “Taylor Glacier is now the coldest known glacier to have persistently flowing water.”