In Antarctica 400 miles west of the South Pole lies a lake that hasn't seen the light of day for 120,000 years.
Lake Whillans is just 2 metres deep and slopes down a hill for 37 miles. It is trapped beneath 800m of ice, and is warmed by heat coming from deep within the planet.
In 2013 scientists took seven days to drill through the ice above the lake. They found a thriving community of single-celled organisms that survived in total darkness, obtaining their energy from the rocks beneath the ice. The finding suggested that life could exist in similar icy lakes elsewhere in the solar system, such as Jupiter's moon Europa.
The microbes living in Lake Whillans are some of the most dramatic examples ever found of life enduring freezing cold temperatures. But they are far from alone in coping with the cold. Life has found ways to carry on in temperatures that would kill a human within minutes.
Antarctica does not hold a monopoly on cold. About 80% of the biosphere is permanently below 5 °C. That may sound unbelievable, but the volume of the oceans is vastly greater than the volume of habitable space on the land, and once you dive past the surface layers, the ocean is all cold.
This means that life on Earth has had plenty of time and opportunity to get used to the chill. As a result, cold-living organisms are more abundant, diverse and widely distributed than any other extreme-living organism.
Antarctica isn't just the coldest place on Earth, it is also one of the windiest
Still, Antarctica takes cold to the extreme. It was here, at the Russian Vostok station, that the lowest temperature on Earth was recorded: -89.2 °C in 1983.
Antarctica is a barren icy desert. Around its coasts temperatures are relatively mild, being close to freezing in the summer and between −10°C and −30°C in the winter. The really cold conditions occur in central Antarctica, where a high plateau rises above the ocean. Here, summer temperatures struggle to get above −20°C and monthly means fall below −60°C in winter.
Antarctica isn't just the coldest place on Earth, it is also one of the windiest. A wind speed of 320 km/h (200 mph) was recorded at the French Dumont d'Urville base in July 1972. The extreme wind chill means that as well as cold, animals must withstand intense dehydration.
Nevertheless, life prevails. In 2014, a new species of tardigrade, a tiny animal that looks like a cross between a worm and a woodlouse, was found on mosses in a crater hollowed out by ancient glaciers.
To live in such a cold place, animals, plants and bacteria face a number of unique challenges.
Cold temperatures even change how an animal's cells and molecules work
Warm-blooded animals must spend vast amounts of energy maintaining their internal body temperature. That requires food and lots of it. Yet in cold places lack of warm sun means less photosynthesis, and less plant life – the very basis of all food chains.
Meanwhile, cold-blooded creatures rely on the small amount of warmth from the Sun to keep their body's reactions going, but are at serious risk from the chill.
Cold temperatures even change how an animal's cells and molecules work.
The fatty membranes that surround all cells become less fluid in the cold. Enzymes, the proteins that enable all the body's reactions, work less well. Water freezes up, becoming ice crystals that pierce and destroy the cells.
What's more, cold winds can lead to cells drying out, and high levels of ultraviolet radiation can damage the very DNA on which life depends.
In response, Antarctica's animals have evolved a host of ways to keep warm.
Most of the warm-blooded animal ones are big and round. Seals and penguins both have a large volume but a small surface area, reducing heat loss from their skin. Penguins lower their surface area further by huddling together in small groups.
What's more, all the large animals are carnivores, as meat is a much more concentrated source of energy than plants.
Despite these adaptations, life in Antarctica is a risky business. Given that the climate is so inhospitable, why do they bother?
The answer lies in the surrounding oceans.
Upwellings of deep ocean water carry nutrients to the surface, and in the summer that means a bounty. It is light for up to 24 hours a day, so microscopic plants called phytoplankton are able to continuously photosynthesize.
Life on Antarctica is almost entirely sustained by the sea
These are then eaten by tiny crustaceans called krill. These in turn are the staple foods for many Antarctic animals.
The end result is that a huge amount of food is available in the Antarctic Ocean for those able to catch it. During the summer, large blue whales can catch and eat 6 tonnes of krill every day for weeks on end.
Life on Antarctica, then, is almost entirely sustained by the sea. But there is one exception: the only Antarctic animal that can survive entirely on land.
The wingless midge Belgica antarctica lives on the rocky outcrops of the Antarctic peninsula. It is the only insect that lives on Antarctica, barring parasites living on larger animals.
It spends most of its 2-year life cycle as a larva encased in a matrix of ice
The midge is subjected to a range of onslaughts including temperature extremes, lack of water, intense ultraviolet exposure and high winds. But it shrugs them all off.
It is able to withstand freezing temperatures, as it spends most of its 2-year life cycle as a larva encased in a matrix of ice. It emerges only in the warmer summer months of December to January, and can go into a stupor for weeks or even months if the temperature drops.
Not having wings ensures that the midge doesn't get buffeted by the wind. Its small size, just 13mm, means it can warm up quickly in the aftermath of a chill.
The midge also has the smallest insect genome ever sequenced, with just 99 million "letters" of DNA compared with 3.2 billion in the human genome. Unlike most animals, its DNA contains very few repeated sequences. This economic approach may help the fly survive.
What's more, the midge have specialised genes that allow it to combat harmful chemicals called reactive oxygen species (ROS), which can damage cells and DNA. Extreme cold, heat, dryness and radiation can all cause cells to produce ROS.
Many animals cope by switching on genes that produce chemicals called antioxidants, which mop up the ROS. In the larvae of Antarctic midges, these genes are constantly on.
Not everything has it this tough. Life in the water is a bit easier.
The seas around Antarctica are often warmer than the land, varying from -2°C to +2°C around the year. Animals like seals and penguins often retreat to the water when it gets too cold.
They can live indefinitely in cold water
However, jumping in the sea can also be perilous. Penguins, whales and seals must actively maintain their internal body temperatures at a range of 35-42°C. Water conducts heat 25 times more effectively than air, so in theory they should lose body heat much quicker in water than air.
To keep themselves warm, seals, penguins and whales have a layer of blubber under their skin. It is remarkably effective at preventing heat loss. If you were to measure the temperature of their skin you would find it nearly identical to the surrounding water, whilst just centimetres beneath the blubber their blood is kept in the vital 35-42°C range.
The animals can also close up blood vessels in their flippers and feet, diverting blood away from the surface and deep into the body, stopping heat loss. As a result, they can live indefinitely in cold water.
Some Antarctic animals do just that.
Antarctic fish have enzymes specially adapted to cope at low temperatures, and many even have anti-freeze in their blood.
These deep-dwelling fish are "supercooled"
The anti-freeze chemicals are made of glycoproteins: proteins with sugars bolted onto them. These large molecules surround any ice crystals that form in the animal's blood or tissues, before their sharp points can puncture cell membranes.
However, anti-freeze proteins are only found in fish that live near the surface and come into contact with floating ice. Fish that live in the depths of the ocean never come into contact with ice crystals, which are only ever found on the ocean surface.
These deep-dwelling fish are "supercooled". Their bodies stay unfrozen despite being in sea water that is colder than the freezing point of their blood and tissues.
If you were to take these fish to the surface, the first ice crystal they found would act as a seed for more to form in their bodies, and they would instantly freeze to death.
Adaptations like these mean that life in the Antarctic seas is rich and diverse. In the dry interior, however, it's a different story.
The Antarctic dry valleys are the coldest and driest deserts on Earth. The only inhabitants are bacteria. Many survive by living under translucent rocks called hypoliths, which protect them from the worst of the stresses.
Ice crystals can also form within the cells
The biting cold threatens the bacteria's cells. Their membranes are made of fats, so they become more rigid, just like when you put butter in the fridge. This hinders the vital transport of molecules in and out of the cells.
Ice crystals can also form within the cells, and the bacteria's enzymes work more slowly.
More insidiously, gases become more soluble at low temperatures. The gases themselves are harmless, but when they dissolve in the cell fluids they can transform into harmful ROS, which can damage cells and attack DNA.
"Bacteria have developed a number of different strategies to cope with these challenges," says microbiologist Pieter de Maayer of Pretoria University in South Africa.
"Their cell membranes contain higher levels of unsaturated fatty acids, which stay fluid when cold. They also produce different versions of proteins and enzymes that are active in cold conditions. They can also switch on genes that are involved in combating reactive oxygen species, and produce proteins that protect DNA against damage at low temperatures."
Some ocean-dwelling bacteria can produce bubbles of gas around themselves
Antarctic bacteria can also produce a number of specialised molecules that protect them against the cold. These include cryoprotectants such as glycine and sucrose, which prevent cells from freezing solid, and anti-freeze proteins that stop the formation of ice crystals.
Other species simply try to avoid the worst of the cold. Some bacteria that live in the ice seek out brine pockets, where the high salt concentration stops them from freezing.
Some ocean-dwelling bacteria can produce bubbles of gas around themselves. They then travel inside the gas bubble to warmer areas.
Others go into a dormant state when it gets too cold. All chemical reactions cease, but when it warms up they can revive themselves.
Antarctica has been cold for millions of years, so the organisms that live there are thoroughly adapted to the chill. But now it is warming up as a result of man-made climate change.
Others will be in trouble because they need the cold to survive
For some bacteria, this might be relatively good news. "Even though a lot of micro-organisms can survive in cold environments, many do a lot better in temperate environments," says de Maayer.
"Bacteria on ice caps, for example, are often exposed to freeze-thaw cycles," he says. They "have to switch on genes coding for proteins and enzymes involved in survival under cold conditions, and switch off non-essential genes to conserve energy. When the temperature rises, this pattern can be reversed."
Others, however, will be in trouble because they need the cold to survive. "These bacteria mostly come from oceans where the temperature remains cold all the time," says de Maayer.
Some could move deeper into the ocean, where the temperature is likely to stay relatively stable. But others may have to survive by "going to sleep" for a long, long time.
The total population is expected to drop by 19%
"Many of these bacteria can cope with rising temperatures by forming spores or going into a torpor state where they are still alive, but have switched off their metabolism, basically going into a low energy-expenditure survival mode," says de Maayer. "This way, they could survive the rising temperatures until the coming of a new ice age."
As for the animals, the outlook is less good. There is simply nowhere for them to go, as the nearest continents – Africa and South America – are already inhabited by a host of different species.
Take emperor penguins. A 2014 study examined what might happen to all 45 known colonies over the course of this century, and predicted that they will all be in decline by 2100. The total population is expected to drop by 19%.
To us, Antarctica is frigid and dangerous – even if we also find it beautiful. But to animals like the emperor penguin and the Antarctic midge, it's the only home they have.