A subglacial lake is a body of water that exists beneath a glacier, usually under an ice cap or ice sheet. These lakes form at the area where ice meets the bedrock below. Here, liquid water can exist even when the temperature is slightly below freezing because the high pressure from the ice above lowers the melting point. Over time, the ice above slowly melts at a rate of a few millimeters each year. Meltwater moves from areas of high water pressure to areas of lower pressure under the ice, collecting in pools that can remain separated from the outside world for millions of years.
Since the first subglacial lakes were discovered under Antarctica’s ice sheet, more than 400 such lakes have been found in Antarctica, beneath Greenland’s ice sheet, and under Iceland’s Vatnajökull ice cap. These lakes hold a large amount of Earth’s liquid freshwater. The total volume of water in Antarctic subglacial lakes alone is estimated to be about 10,000 cubic kilometers, which is roughly 15% of all liquid freshwater on Earth.
Subglacial lakes are isolated from Earth’s atmosphere and are shaped by interactions between ice, water, sediment, and living organisms. They contain active communities of microbes that can survive in extremely cold and nutrient-poor environments. These microbes help drive chemical processes that do not rely on sunlight. Scientists study these lakes and their life forms because they provide clues about how life might exist in extreme environments, such as those found on other planets.
Physical characteristics
The water in subglacial lakes stays liquid because heat from the Earth's interior balances the heat lost through the ice above. The heavy weight of the glacier above lowers the temperature at which water freezes, making it below 0 °C. The top of a subglacial lake is at the level where the temperature changes with depth meet the temperature at which pressure causes melting. In Lake Vostok, the largest subglacial lake in Antarctica, the ice above the lake is much thicker than the ice around it. Lakes with very high salt levels remain liquid because salt lowers the freezing point of water.
Not all lakes with ice covering them are subglacial. Some, like Lake Bonney and Lake Hoare in Antarctica's McMurdo Dry Valleys and Lake Hodgson, have regular lake ice instead of being covered by glaciers.
The water level in a subglacial lake can be much higher than the ground beneath it. A subglacial lake could even exist on a hill if the ice above is thin enough to form a seal that holds the water in place. This water level can be imagined as the height of water in a hole drilled through the ice into the lake. It is similar to the height at which a floating ice shelf would sit. The top of the lake can be thought of as an ice shelf that is fixed in place along its edges, which is why it is sometimes called a captured ice shelf. As the ice moves, it enters the lake at the floating level and exits at the grounding line, where the ice meets the ground.
A hydrostatic seal forms when ice around a lake is so high that water pressure pushes it down into impermeable ground. Water beneath this ice is then forced back into the lake by the seal. In Lake Vostok, the ice rim is about 7 meters high, while the water level is about 3 kilometers above the lake's top. If the seal is broken when the water level is high, water may flow out rapidly in a jökulhlaup. As melting increases the flow, water moves faster unless other factors speed it up further. Because of the high pressure from water in some subglacial lakes, jökulhlaups can release water very quickly. In Iceland, volcanic activity can melt ice and cause sudden floods by breaking ice dams and lake seals.
The effect of subglacial lakes on ice movement is not fully understood. In Greenland, water beneath the ice increases the speed of ice movement in complex ways. In Antarctica, lakes under the Recovery Glacier may influence the movement of nearby ice streams. A small increase in the speed of the Byrd Glacier in East Antarctica might have been caused by a sudden release of water from beneath the ice. Water movement is observed in areas where ice streams change speed or direction over long periods, showing that water may flow over the edge where ice meets the ground.
History and expeditions
Russian revolutionary and scientist Peter A. Kropotkin first suggested that liquid freshwater might exist under the Antarctic Ice Sheet at the end of the 19th century. He believed that heat from the Earth’s interior could warm the ice enough to melt it, even though the temperature would still be below freezing. Later, Russian glaciologist Igor A. Zotikov used theoretical analysis to explain how melting at the bottom of ice sheets might reduce the size of Antarctic ice. By 2019, scientists had identified more than 400 subglacial lakes in Antarctica, and more may exist. Subglacial lakes have also been found in Greenland, Iceland, and northern Canada.
Scientific progress in Antarctica has happened during several major periods of teamwork, such as the four International Polar Years (IPY) in 1882–1883, 1932–1933, 1957–1958, and 2007–2008. The success of the 1957–1958 IPY led to the creation of the Scientific Committee on Antarctic Research (SCAR) and the Antarctic Treaty System, which helped scientists develop better methods to study subglacial lakes.
In 1959 and 1964, Russian geographer Andrey P. Kapitsa used seismic sounding to map the layers of rock and ice beneath Vostok Station in Antarctica. His goal was to study the Antarctic Ice Sheet. Data from these surveys was used 30 years later to discover Lake Vostok, a subglacial lake.
Starting in the late 1950s, English scientists Stan Evans and Gordon Robin used a technique called radio-echo sounding (RES) to measure the thickness of ice. Subglacial lakes are identified by RES data as smooth, flat surfaces that reflect sound waves. In the late 1960s, they placed RES instruments on airplanes to collect data about the Antarctic Ice Sheet. Between 1971 and 1979, RES was used to map large areas of the ice sheet. The RES method involves drilling 50-meter-deep holes in ice to improve signal quality. A small explosion sends sound waves through the ice, which reflect off the bottom and are recorded. The time it takes for the sound to return is used to calculate depth. RES data can identify subglacial lakes through three signs: 1) strong reflections from the ice base, 2) consistent echoes showing a smooth surface, and 3) flat, horizontal features with less than 1% slope. Using RES, Kapitsa and his team found 17 subglacial lakes. RES also helped discover the first subglacial lake in Greenland and showed that these lakes are connected.
Between 1971 and 1979, RES was used again to map the Antarctic Ice Sheet. A collaboration between the United States, the United Kingdom, and Denmark surveyed about 40% of East Antarctica and 80% of West Antarctica, improving understanding of ice flow and the subglacial landscape.
In the early 1990s, data from the European Remote-Sensing Satellite (ERS-1) mapped Antarctica down to 82 degrees south. This data revealed a flat surface near Lake Vostok and helped scientists understand the distribution of subglacial lakes.
In 2005, Laurence Gray and other scientists used data from the RADARSAT satellite to study changes in ice surfaces, which suggested the presence of active subglacial lakes with moving water.
Between 2003 and 2009, NASA’s ICESat satellite mapped the first large-scale view of active subglacial lakes in Antarctica. In 2009, Lake Cook was identified as the most active subglacial lake. Other satellites, including ICESat, CryoSat-2, and SPOT5, have been used to monitor these lakes.
Gray et al. (2005) used RADARSAT data to show that changes in ice surfaces could indicate subglacial lakes filling or emptying. Wingham et al. (2006) used ERS-1 data to find evidence of water moving between lakes. NASA’s ICESat satellite helped confirm these findings, and by 2009, scientists had identified 124 active subglacial lakes. The discovery that lakes are connected raised concerns about contamination during drilling.
Surveys by SPRI-NSF-TUD until the 1970s first mapped many subglacial lakes. Later studies, such as those by Carter et al. (2007), used RES data to classify different types of subglacial lakes.
In March 2010, the sixth international conference on subglacial lakes was held in Baltimore. Scientists discussed tools for drilling through ice, such as hot-water drills and methods to collect water and sediment samples while protecting the environment. A code of conduct for drilling was created by SCAR and approved in 2011. By 2011, three drilling missions were planned.
In February 2012, Russian scientists reached Lake Vostok for the first time. Water from the lake froze in the borehole during winter, and samples of frozen water were collected in 2013. In December 2012, British scientists tried to reach Lake Ellsworth but had to stop due to equipment problems. In January 2013, the U.S.-led WISSARD expedition collected samples from Lake Whillans to study microbes. In December 2018, the SALSA team reached Lake Mercer after drilling through 1,067 meters of ice. They collected water and sediment samples from the lake.
Distribution
Most of the nearly 400 subglacial lakes in Antarctica are found near ice divides, which are areas where large drainage basins are covered by ice sheets. The largest lake is Lake Vostok, while other lakes, such as Lake Concordia and Aurora Lake, are also large. More lakes are being discovered near ice streams. A satellite survey from 1995 to 2003 showed unusual patterns in ice sheet height, suggesting that East Antarctic lakes are connected by a system that moves meltwater from the bottom of the ice sheet through subglacial streams.
The largest Antarctic subglacial lakes are grouped in the Dome C-Vostok area of East Antarctica. This may be due to thick ice that acts as insulation and uneven land shapes beneath the ice. In West Antarctica, Lake Ellsworth is located in the Ellsworth Mountains and is smaller and shallower. The Siple Coast Ice Streams in West Antarctica cover many small lakes, including Whillans, Engelhardt, Mercer, and Conway, as well as their smaller neighbors, Lower Conway and Lower Mercer. Retreating ice at the edges of the Antarctic Ice Sheet has exposed former lakes, such as Progress Lake in East Antarctica and Hodgson Lake near the Antarctic Peninsula.
Subglacial lakes beneath the Greenland Ice Sheet were only recently discovered. Radio-echo sounding has identified two lakes in the northwest part of the ice sheet. These lakes are likely filled by water from nearby surface lakes, not from melting at the bottom of the ice. Another potential lake was found near the southwestern edge of the ice sheet, where a circular depression suggests recent drainage caused by climate warming. This drainage, along with heat from surface meltwater, may affect how quickly the ice flows and how the Greenland Ice Sheet behaves overall.
Much of Iceland is volcanically active, producing large amounts of meltwater beneath its ice caps. This meltwater accumulates in basins and ice cauldrons, forming subglacial lakes. These lakes help transfer heat from geothermal vents to the bottom of the ice caps, melting ice that replaces water lost through drainage. Most Icelandic subglacial lakes are found under the Vatnajökull and Mýrdalsjökull ice caps, where volcanic activity creates permanent depressions filled with meltwater. Sudden drainage from these lakes is a known risk in Iceland, as volcanic eruptions can generate enough meltwater to break ice dams and cause flooding.
Grímsvötn is the most famous subglacial lake under the Vatnajökull ice cap. Other lakes are found in the Skatfá, Pálsfjall, and Kverkfjöll cauldrons. Grímsvötn’s ice dam remained intact until 1996, when meltwater from the Gjálp eruption caused it to rise.
The Mýrdalsjökull ice cap, another area with subglacial lakes, sits above an active volcanic caldera in the southern part of the Katla volcanic system. Heat from below the ice cap is believed to have formed at least 12 small depressions within an area defined by three major drainage basins. Many of these depressions hold subglacial lakes that can drain suddenly during eruptions, posing a danger to nearby people.
Until recently, only old subglacial lakes from the last ice age were found in Canada. These ancient lakes likely formed in valleys before the Laurentide Ice Sheet moved during the Last Glacial Maximum. However, two subglacial lakes were recently discovered under the Devon Ice Cap in Nunavut, Canada, using radar. These lakes are likely very salty due to interactions with salt-rich bedrock and are more isolated than similar lakes in Antarctica.
Ecology
Subglacial lakes are different from surface lakes because they are not connected to Earth's atmosphere and do not receive sunlight. These lakes are described as ultra-oligotrophic, meaning they have very low levels of nutrients needed for life. Even though these lakes are extremely cold, have little nutrients, high pressure, and no light, they still support thousands of types of microscopic life and some signs of more complex life. Professor John Priscu, a scientist who studies polar lakes, has called Antarctica's subglacial ecosystems "our planet's largest wetland."
Microorganisms and weathering processes create chemical reactions that support a unique food web, helping to cycle nutrients and energy in subglacial lakes. Since there is no sunlight in these dark lakes, food webs rely on chemosynthesis and the use of ancient organic carbon that was deposited before glaciers formed. Nutrients can enter subglacial lakes through the interface between glacier ice and lake water, through hydrologic connections, and from the weathering of subglacial sediments.
Because few subglacial lakes have been studied directly, most knowledge about their chemical and biological makeup comes from a small number of samples, mostly from Antarctica. Scientists also use ice that forms when lake water refreezes at the bottom of glaciers (called accretion ice) to infer information about unsampled lakes. These inferences assume that the chemical makeup of accretion ice is similar to the lake water it formed from. Studies have found that subglacial lakes have a wide range of chemical conditions, from oxygen-rich upper layers to oxygen-poor, sulfur-rich lower layers. Despite their low nutrient levels, these lakes and their sediments may contain significant amounts of nutrients, especially carbon.
Air clathrates trapped in glacial ice are the main source of oxygen for subglacial lakes. When the bottom layer of ice over a lake melts, clathrates are released, freeing oxygen and other gases for use by microbes. In some lakes, cycles of freezing and melting at the lake-ice interface can increase oxygen levels in upper lake water to 50 times higher than in typical surface waters.
Melting glacial ice above subglacial lakes also adds minerals containing iron, nitrogen, and phosphorus, along with dissolved organic carbon and bacterial cells to the water below.
Because air clathrates from melting ice are the main source of oxygen, oxygen levels in subglacial lakes generally decrease with depth if water turnover is slow. Oxygen-rich or slightly low-oxygen water is often found near the glacier-lake interface, while oxygen-poor conditions dominate in the lake's interior and sediments due to microbial respiration. In some lakes, microbes may consume all the oxygen, creating an oxygen-free environment until new oxygen-rich water flows in from connected areas. The addition of oxygen from ice melt and its use by microbes can create changes in oxygen levels, with aerobic processes like nitrification occurring in upper waters and anaerobic processes in lower, oxygen-free layers.
Subglacial lakes have low concentrations of solutes such as sodium, sulfate, and carbonates compared to surface lakes. These solutes enter the water from melting ice and weathering of sediments. Despite their low solute levels, the large volume of subglacial water makes them important contributors of solutes, especially iron, to surrounding oceans. Subglacial outflow from the Antarctic Ice Sheet is estimated to add similar amounts of solutes to the Southern Ocean as some of the world's largest rivers.
The movement of water between subglacial lakes and streams under ice sheets influences biogeochemical processes, affecting microbial habitats and oxygen and nutrient levels. Hydrologic connections also change how long water stays in a subglacial lake. Longer water residence times, such as those found beneath the interior Antarctic Ice Sheet, allow more contact between water and solute sources, leading to greater solute accumulation. Estimated water residence times in studied subglacial lakes range from about 13,000 years in Lake Vostok to just decades in Lake Whillans.
The shape of subglacial lakes can affect their hydrology and circulation. Thicker ice above lakes leads to faster melting, while thinner ice allows lake water to refreeze. These differences in melting and freezing rates create internal water movement and circulation of solutes, heat, and microbes, which vary by region.
Subglacial sediments are mainly made of glacial till formed by physical weathering of bedrock. These sediments are oxygen-poor because microbes consume oxygen, especially during the oxidation of sulfide minerals. Sulfide minerals form when glaciers weather bedrock, and these sulfides are then converted to sulfate by aerobic or anaerobic bacteria, which may use iron for respiration when oxygen is unavailable.
The products of sulfide oxidation can increase the chemical weathering of carbonate and silicate minerals in subglacial sediments, especially in lakes with long water residence times. Weathering of these minerals also releases ions such as potassium, magnesium, sodium, and calcium into lake water.
Other processes in anoxic subglacial sediments include denitrification, iron reduction, sulfate reduction, and methanogenesis.
Subglacial sedimentary basins under the Antarctic Ice Sheet are estimated to hold about 21,000 petagrams of organic carbon, mostly from ancient marine sediments. This amount is more than 10 times the organic carbon in Arctic permafrost and may be comparable to reactive carbon in modern ocean sediments, making subglacial sediments an important but understudied part of the global carbon cycle. If ice sheets collapse, this organic carbon could be released into the atmosphere, potentially accelerating climate change.
Microbes in subglacial lakes likely influence the form and fate of sediment organic carbon. In oxygen-poor sediments, organic carbon can be used by archaea for methanogenesis, creating large amounts of methane trapped in sediments. Methane has been found in subglacial Lake Whillans, and experiments show that methanogenic archaea can be active in sediments beneath both Antarctic and Arctic ice sheets.
Implications for extraterrestrial life
Subglacial lakes are environments similar to ice-covered water on other planets, making them important for studying the possibility of life beyond Earth. Astrobiology is the scientific study of life's potential to exist outside Earth. Finding microbes that can survive in extreme conditions in Earth's subglacial lakes may help scientists understand if life could exist in similar places on other celestial bodies. These lakes also help researchers prepare for studying distant, difficult-to-reach locations that must be protected from contamination by Earth-based life.
Jupiter's moon Europa and Saturn's moon Enceladus are key places in the search for life beyond Earth. Europa has a large ocean covered by ice, and Enceladus is believed to have a hidden ocean beneath its surface. Observations of water vapor plumes from Enceladus show that hydrogen is produced underground, which may indicate chemical reactions involving iron-rich minerals and organic materials.
In 2018, scientists discovered a subglacial lake on Mars using radar equipment on the Mars Express spacecraft. This body of water was found under Mars' South Polar Layered Deposits and is thought to have formed when heat from the planet's interior melted ice beneath the surface.