Blood Falls is a blood-red waterfall in Antarctica.

Antarctica is bleeding. Not only are its glaciers melting because of global warming. There is also Blood Falls – a waterfall where red-colored water seeps out from the glacial ice.

With the mystery of the red color solved some questions remain. Where does the water come from? Where does the iron come from? And why is the water not frozen, even though the average temperature in the area is -17° Celsius (1.4° Fahrenheit)?

Blood Falls flows out of the mouth of Taylor Glacier, but it does not consist of melt water from the glacier as one might think. Instead, the water comes from an ancient lake that was formed 5 millions years ago by ocean water that flooded East Antarctica. Two millions years ago, glaciers were formed over the lake, trapping it underneath.

When the water on the surface of the subglacial lake began to freeze, the salt concentration in the remaining water became higher and higher. Today, the water is three times as salty as the ocean, which means that it does not freeze despite temperatures dipping below 0° Celsius (32° Fahrenheit).

The iron in the water is thought to come from particles that enters the lake through the scraping motion of Taylor Glacier – iron is common in the Antarctic bedrock. Also, microbes that feed on iron and sulfur live here. These are thought to add iron to the water as they erode the iron-rich bedrock around the lake.

In 2017, researchers revealed the answer to how water from the subglacial lake can be transported through the glacier. With the help of radar scanning, they found a network of subglacial rivers flowing through cracks in the glacier. The salty water from the source lake has a freezing point lower than the glacier and releases heat when it freezes – something called latent heat. The source water that comes into contact with the glacier freezes, while the latent heat melts the glacial ice, enabling rivers to flow.

Since some of the source water have frozen on the way through the glacier, the water that reaches the glacier’s mouth has an even higher concentration of salt and iron. The presence of salt also increases the rate of the reaction between iron and oxygen, making Blood Falls even redder.

Blood Falls flows out onto a frozen lake in Taylor Valley, which is one of the McMurdo Dry Valleys – a series of valleys in Antarctica which despite the cold temperatures are ice-free due to the area’s dry weather.