A Most Unusual Lake

Scientists estimate that Earth is home to more than 100 million lakes. Among the most unusual is Lake Unter-See, one of Antarctica’s largest and deepest surface lakes, known for its distinctive water chemistry. Its ice-covered waters have exceptionally high levels of dissolved oxygen, low dissolved carbon dioxide, and a strongly alkaline (basic) pH.

The OLI (Operational Land Imager) on Landsat 9 captured this image on February 16, 2026, during the Antarctic summer. Most of the lake’s water comes from seasonal meltwater draining from the margins of the nearby Anuchin Glacier, which flows south from the Gruber Mountains in Queen Maud Land.

With mean annual temperatures of about minus 10 degrees Celsius (14 degrees Fahrenheit), Lake Unter-See remains frozen year-round, its waters sealed beneath several meters of ice. Sunlight penetrates the ice and warms the water below, but the cold surface and strong winds drive evaporation and sublimation, preventing significant surface melting. The lake’s maximum depth is thought to reach nearly 170 meters (558 feet).

The lake’s water chemistry is unusual partly because it is one of the only perennially frozen lakes with a community of large, conical stromatolites. The layered microbial reef structures grow slowly upward as photosynthetic microbes—primarily cyanobacteria—trap sediment on their sticky surfaces and form calcium carbonate mineral crusts. These conical stromatolites—as well as pinnacle and flat forms of the microbial communities—release oxygen that becomes trapped under the ice, increasing its concentration in the lake.

Lake Unter-See’s stromatolites, discovered by SETI geobiologist Dale Andersen and colleagues in 2011, offer a glimpse into a time more than 3 billion years ago, when microbes were the only form of life on Earth. The formations are thought to be modern, living examples of the organisms that likely produced some of Earth’s oldest fossils—stromatolites found in places such as southwestern Greenland and western Australia.

Some Antarctic lakes, such as Lake Joyce in the McMurdo Dry Valleys, contain conical stromatolites, but they reach only a few centimeters tall. By contrast, the formations in Lake Unter-See tower up to half a meter. Scientists think Unter-See’s stromatolites grow unusually tall because they are sheltered from tides and waves beneath permanent ice, live in exceptionally clear waters with little sediment, grow toward limited light, and face little grazing. The lake’s largest creatures are tardigrades—microscopic “water bear” invertebrates known for their ability to survive in extreme environments.

Astrobiologists also point to the lake as a possible analog for the type of environment where life might have formed or survived on icy moons with oceans such as Europa and Enceladus, or perhaps on Mars, which has ice caps and glaciers.

Yet despite its seemingly stable conditions, Lake Unter-See occasionally experiences abrupt changes. During fieldwork in 2019, researchers observed an increase in the lake’s water levels. The team, led by scientists at the University of Ottawa, later analyzed elevation data from NASA’s ICESat-2 (Ice, Cloud, and Land Elevation Satellite-2) and confirmed a 2-meter rise was caused by a glacial lake outburst flood from nearby Lake Ober-See.

The University of Ottawa team also showed that the outburst flood had released 17.5 million cubic meters of meltwater, altering Unter-See’s pH and replenishing it with carbon dioxide-rich waters that likely enhanced the productivity of the lake’s microbial life. The scientists noted that similar periodic flooding may provide “biological stimuli to other carbon dioxide-depleted Antarctic ecosystems and perhaps even icy lakes on early Mars.”

NASA Earth Observatory image by Michala Garrison, using Landsat data from the U.S. Geological Survey. Story by Adam Voiland.