In Antarctica, ice is never just ice. It dominates the landscape, and for much of the year it dominates the sea too. The numbers alone never quite communicate its epic quality.Over 99% of Antarctica is covered in ice, much of it is more than a mile thick, and every polar winter the sea around it freezes to such an extent that the continent effectively doubles in size.
It’s only when in Antarctica you start to realise that ice here is all about sensation: the first sighting of an iceberg after crossing the Drake Passage, the hushed tinkling of brash ice as you carve through it in a zodiac, or a glassy chunk of glacial ice fished from the water, thousands of years old.
To help navigate your way around this frozen world, and learn how to tell an ice sheet from an ice shelf, and how greasy ice can turn into pancake ice, we’ve put together this guide to Antarctica’s incredible ice.
Icebergs
Icebergs are the heralds of Antarctica: when sailing across the Drake Passage from South America, it’s always a thrill when the first iceberg of the trip looms into view on the horizon (we’ve even been expedition cruise ships that run a sweepstake to guess the hour of the first sighting).
Icebergs are enormous chunks of ice that have broken off a glacier or ice shelf. The biggest icebergs can be unfathomably huge: in 2025, satellites tracked A23a, the largest iceberg in the world, as it drifted from the Weddell Sea to eventually ground itself close to South Georgia. It measured around 3,500 square kilometres (1,400 sq mi) – more than twice the size of London. Since 90% of an iceberg sits under the water, the dimensions under the surface were almost as staggering: with cliffs reaching 40 metres high, A23A hid almost 360 metres of ice below the waves.
Most icebergs encountered on an Antarctic cruise are far smaller, of course. As a rule, anything 5 metres above the surface is classified as an iceberg. Smaller than this — down to around a metre — and the ice is charmingly referred to as a bergy bit.
Less charming are the growlers, which is what you get when there’s just a metre of ice showing above the water. These are generally too small to be picked up by a ship’s radar, but still chunky enough to cause potential hazards. They’re one reason why expedition cruise ships are equipped with the most advanced navigation equipment and have the best navigators in the business on the bridge when sailing in polar waters.
Sea ice
As icebergs were born from glaciers, the products of ancient snowfall, they’re made of freshwater. When Captain Cook led the first recorded crossing of the Antarctic Circle in 1773, his ships were able to sustain themselves with water by fishing broken chunks of icebergs from the sea. But not all of Antarctica’s ice makes good drinking water: most of it is salty.
Every year, Antarctica undergoes the greatest environmental change on the planet. When the sun disappears for the winter, the sea around Antarctica freezes, effectively doubling the size of the continent. This sea ice — usually just a couple of metres thick — is of enormous importance to the Southern Ocean and our planet’s climate.
By limiting the amount of sunlight that reaches the sea, sea ice regulates the growth of phytoplankton, which is the basis of all life in the ocean, as well as providing a nursery for that essential ecosystem inhabitant, Antarctic krill. Where the sea absorbs the sun’s heat, sea ice reflects it back, and then helps keep the ocean and atmosphere cool as it melts every season. In recent years, there has been a big decrease in the extent of Antarctic sea ice, for reasons that scientists are still trying to unravel.
New ice and old ice
If you travel to the Antarctic Peninsula in the spring and early summer, you’ll experience the last of the sea ice on the Antarctic Peninsula, which by this time has largely melted into brash ice – loose accumulations of small sea ice. Taking a zodiac cruise through brash ice is a real treat, hearing the ice brush against its hull and seeing it gently ripple like a carpet around you. Any larger surviving large chunks are called rotten ice, which are honeycombed with meltwater and not long for the world.
Mixed in with the brash ice will undoubtedly be plenty of bergy bits, growlers and small pieces of freshwater glacial ice. These can be extraordinary shades of blue – and the bluer the colour the older the ice. Over thousands of years, air is slowly squeezed out of the glacier ice, changing the way it refracts light, hence its colour. Pluck a piece of glacier ice from the water and hold it to your ears and you can listen to it melt with tiny pops and pings as thousand-year-old air escapes into the atmosphere. The clearest and glassiest chunks are often collected as trophies for the ship’s bar: since they would naturally melt anyway, why not take a piece to cool a cocktail in the evening?
By contrast, if you’re in the Peninsula at the very end of the season, you may get to witness the magic of sea ice forming in front of your very eyes. As the sea cools, the first phase is the formation of frazil ice, tiny specks of ice that begin to form a loose slush on the surface. This coagulates into thin layers called grease ice, and as it thickens, it becomes pancake ice. This looks as the name suggests, and is as thick as your hand and up to three metres across. As the sea continues to freeze, large floes form, eventually joining together into massive sheets, or forming a giant mosaic of pack ice.
Most sea ice is born and disappears in a single year, but in some parts of Antarctica, such as the Weddell Sea, it can persist over several years. This old ice becomes increasingly thick, and requires an icebreaker to penetrate. It was the thick pack ice of the Weddell Sea that caught Shackleton’s ship Endurance in 1915, crushing his hopes of crossing Antarctica.
Ice sheets
When the first explorers started to probe Antarctica at the end of the 19th Century, it was unclear whether it was a new continent or a collection of islands. The reason for this uncertainty was the enormous ice sheet that covers everything.
As with everything Antarctic, the statistics are hard to comprehend. The Antarctic Ice Sheet is nearly 14 million square kilometres (5.4 million square miles), and holds about 60% of all the fresh water on the planet in its giant mass. If that was to melt, it would cause global sea levels to rise by 70 metres.
There are two ice sheets, either side of the Transantarctic Mountains that run the length of the continent. The East Antarctic Ice Sheet lies on the land mass of the continent, but much of the West Antarctic Ice Sheet (in the half of the continent that includes the Weddell Sea and the Peninsula) is so thick that its bottom sits 2,500 metres (8,200 feet) below sea level, on what was be the sea bed if it wasn’t frozen.
The ice sheet forms a white reflective dome over Antarctica, helping keep the continent cold. Snow falling on the ice sheet never melts, even in summer, but builds up over the years, centuries and even millennia, eventually transforming into thick glacier ice.
Even in summer Antarctic temperatures are below 0°C and so frost and snow crystals that gather on the surface of the ice sheet do not melt, but accumulate year-by-year. As these crystals are buried, the weight of the ice above presses them together. Eventually, they are transformed into dense and impermeable glacial ice. The oldest ice surveyed on the East Antarctic ice sheet is one million years old, which scientists are using to better understand our ancient climate and what it can tell us about our changing world today.
Ice shelves
Where Antarctica’s ice sheets meet the ocean at the continental edge, you find ice shelves. These are immense sheets of ice that, instead of sitting on bed rock, find themselves floating in water. More than 75% of Antarctica’s coastline is surrounded by ice shelves.
Although Antarctica is fringed with ice shelves, two are particularly famous. The first is the Ross Ice Shelf on the edge of the Ross Sea, encountered by explorers like Shackleton, Amundsen, Scott and James Clark Ross. Ross gave his name to both the sea and the ice shelf, which he knew simply as ‘The Barrier’ due to its impenetrable nature. The Ross Ice Shelf is slightly larger than France.
In West Antarctica, the Filchner–Ronne Ice Shelf protrudes into the Weddell Sea. The A23a iceberg was calved from this shelf back in 1986, and spent nearly 40 years grounded on the sea bed before floating free into the open ocean. Seeing the immense tabular icebergs that are born here from the Filchner–Ronne Ice Shelf is one of the most magical parts of a visit to the Weddell Sea.
As ice shelves are exposed to both the atmosphere and the melting effects of the sea, they are the part of Antarctica most acutely sensitive to climate change. A number of ice shelves in the Antarctic Peninsula are currently in retreat. As the ice shelves float on water, this has little impact on sea level, but can have upstream effects by removing protection from the sea on glaciers draining from the edge of grounded ice sheets.
When it comes to Antarctica, ice is never just ice – it tells stories of the continent’s vast history, and reflects the incredible seasonal change the region undergoes every year. So if you’re heading to Antarctica expecting just to fall in love the penguins, be ready to be swept away by the extraordinary beauty and variation of its frozen landscapes.
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