A large glacier in Antarctica produces a bright red river that oozes out of the ice, aptly named Blood Falls. Why the reddish water pours out of Antarctica’s Taylor Glacier and into Lake Bonney puzzled scientists for decades.

The phenomenon was first discovered by geologist Griffith Taylor in 1911. At the time, he thought that red algae living in the water was responsible for the water’s striking red hue.

More than a century later, scientists found what causes the bloody river: iron salts seeping out of the ice that turn red when they make contact with the air.

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In a 2017 study, scientists found that Taylor Glacier formed roughly 2 million years ago, trapping a saltwater lake under it. Millions of years later, the ancient lake has reached the edge of the glacier, squeezing out salt water.

Read more: yahoo