The McMurdo Dry Valleys of Antarctica are among the strangest places on Earth. Despite being on the coldest continent, they are almost free of ice and snow. Fierce winds pour down from the mountains, sweeping the valleys clear. The landscape of bare rock and gravel is so harsh that NASA has used it to test equipment for Mars.

In one of these valleys, the Taylor Glacier ends in a wall of white ice. And from its face, a stain of deep red spills down onto the frozen lake below, like a wound.

Explorers called it Blood Falls.

A red mystery

The Australian geologist Griffith Taylor discovered the red outflow in 1911, during the British Antarctic Expedition led by Robert Falcon Scott. The glacier was later named after him.

At first, scientists thought the colour came from red algae, which can tint snow pink and red in some cold places. It was a reasonable guess. But it turned out to be wrong.

Rust from the deep

The water that flows out at Blood Falls is brine, extremely salty water. It is rich in dissolved iron. Deep inside and beneath the glacier, where there is no oxygen, the iron stays dissolved and the water is clear.

When the brine reaches the surface and meets the air, the iron reacts with oxygen and turns into iron oxide — rust. The water stains the ice orange-red.

Where does the water come from?

Scientists think the brine is the remains of ancient seawater. Millions of years ago, when sea levels were higher, the sea reached into the Dry Valleys. As the climate changed and glaciers advanced, some of that water was trapped beneath the ice. Over time, it became concentrated into salty brine, sealed off from the outside world.

Estimates of how long the water has been trapped range up to around a million years or more.

In 2017, researchers used radar to look inside the Taylor Glacier. They found a network of brine channels and pockets within and beneath the ice, feeding the outflow at Blood Falls. It was the first time scientists had confirmed that liquid water could flow persistently inside such a cold glacier.

Life in the dark

The most surprising discovery at Blood Falls is that the brine is alive.

In 2009, a team led by the microbiologist Jill Mikucki reported finding a community of microbes living in the brine. They have no sunlight. There is very little oxygen. The water is cold, salty and dark. Yet the microbes survive, apparently by using iron and sulfur compounds to get energy from the small amounts of organic matter in the water.

Why it matters

Blood Falls is studied by scientists interested in more than Antarctica. If microbes can survive in dark, salty, oxygen-poor water trapped beneath ice on Earth, perhaps similar life could survive in similar places elsewhere in the solar system.

Mars has evidence of ancient water and salty deposits, and some researchers think briny water may still exist beneath its surface. Jupiter's moon Europa and Saturn's moon Enceladus are thought to have oceans of salty water beneath their icy crusts.

Visiting

Blood Falls is extremely remote. It can be reached only by helicopter from research stations, and it is protected under the Antarctic Treaty system. Few people will ever see it in person.

But photographs of the red stain against white ice have made it one of Antarctica's most famous sights — a reminder that even the coldest, most hostile places on Earth can hide liquid water and life.