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Scientists believe our planet has more than 100 million lakes. Most of these lakes look very similar to one another. However, some have strange and unique features. One of the most unusual is Lake Unter-See in Antarctica. It is one of the largest and deepest surface lakes on the entire continent. What makes this lake truly special is the chemistry of its water.
Although the lake is covered by thick ice, the water trapped inside has exceptionally high levels of dissolved oxygen and methane. At the same time, the water has low amounts of dissolved carbon dioxide. It also has a high pH level, which means the water is very basic. These unusual conditions create a hidden world that is unlike any other lake on Earth.
The Operational Land Imager (OLI) on the Landsat 9 satellite took a picture of this lake on February 16, 2026. This image was captured during the Antarctic summer. Even though it is summer in this part of the world, the climate remains extremely cold. The average annual temperature is about minus 10 degrees Celsius, which is 14 degrees Fahrenheit. Because of this cold weather, Lake Unter-See stays frozen all year long. Its waters are tightly sealed beneath a layer of ice that is between 2 to 4 meters thick. This means the ice is roughly 7 to 13 feet deep.
Sunlight can pass through this thick ice and reach the water below. This sunlight warms the water, which causes the submerged face of the Anuchin Glacier to melt where it enters the lake. However, the surface of the lake stays very cold. Strong winds also blow across the ice, causing evaporation and sublimation. Sublimation is a process where ice turns directly into water vapor without becoming liquid first. These processes prevent significant melting of the surface ice. The lake is very deep, with a maximum depth of nearly 170 meters, or 558 feet.
Hidden beneath the thick ice is one of the lake's most remarkable features. Scientists have discovered large, conical structures spread across the lake floor. These structures are called microbialites. They are formed by photosynthetic microbes that trap and bind sediment together. Researchers first discovered these formations in 2008. The lead geobiologist, Dale Andersen, and his colleagues found them during their studies.
The formations are thought to be modern, living examples. They are very similar to some of Earth's oldest fossils, known as stromatolites. Stromatolites are found in places like southwestern Greenland and Western Australia. These structures offer a glimpse into a time more than 3 billion years ago. That was a time when microbes were the only form of life on our planet.
The formations in Lake Unter-See are quite large compared to similar structures found in other places on Earth. Dale Andersen explained the unique nature of these structures. "Unter-See's microbialites are unlithified, finely laminated structures that rise up to 70 centimeters above the lake floor and, to date, have not been reported in any other modern environment," Andersen said. The microbes that build these formations are primarily cyanobacteria. They likely benefit from what Andersen described as the lake's "restricted" and "low-disturbance" conditions. In this environment, the largest animals are microscopic. They include rotifers, nematodes, and tardigrades, which are tiny organisms often found in water.
Astrobiologists study Lake Unter-See because it might tell us about life on other planets. They look to the lake as a possible analog for environments where life might exist on other worlds. An analog is a model that behaves in a similar way to something else. The lake has a dark, stratified, anoxic south basin. "Anoxic" means the water has no oxygen. This area supports a diverse microbial ecosystem.
Scientists have treated this small part of the lake as a model for the subsurface ocean on Saturn's moon, Enceladus. Enceladus is a lightless environment that contains dissolved hydrogen, methane, carbon dioxide, and ammonia. On Enceladus, microbial life, if it is present, would likely rely on chemistry rather than sunlight for energy. The ice-sealed waters of Lake Unter-See also make it a valuable analog for the ice-covered lakes that scientists think existed on early Mars. This helps researchers understand how life might survive in frozen, dark places in the solar system.
Despite its seemingly stable conditions, the lake occasionally experiences abrupt and sudden changes. During fieldwork in 2019, Andersen and his colleagues observed a rapid increase in the water levels of Lake Unter-See. This change was unexpected and dramatic. The team later analyzed elevation data from NASA's ICESat-2 satellite, which stands for Ice, Cloud, and Land Elevation Satellite-2. They confirmed that the water level rose by 2 meters. The cause was a glacial lake outburst flood from the nearby Lake Ober-See. A flood like this happens when a lake dammed by ice breaks open suddenly. Time-lapse images from cameras on the ground also recorded this event. In situ measurements, which are measurements taken directly at the scene, were made afterward. These measurements documented the effects of the flood within the lake.
The team concluded that the outburst flood had released about 17.5 million cubic meters of meltwater. This massive amount of water altered the pH of Lake Unter-See. It also replenished the normally carbon-depleted system with carbon dioxide-rich waters. Adding carbon dioxide likely enhanced microbial productivity, meaning the microbes grew faster. The scientists noted that similar periodic flooding may provide "biological stimuli to other carbon dioxide-depleted Antarctic ecosystems." This suggests that such events could be important for life in icy lakes on early Mars as well. These floods might trigger biological activity in places where life is struggling to survive due to a lack of carbon dioxide.
The story of Lake Unter-See shows how extreme environments on Earth can teach us about the history of life here and the potential for life elsewhere. The unique chemistry, the ancient microbial structures, and the sudden floods all contribute to our understanding of the universe. By studying this frozen lake, scientists hope to unlock secrets about the origins of life and the future of exploration on other worlds.