Under nearly half a mile of ice, a remote-controlled yellow submarine makes its way through dark water. The scientists directing it struggle to make anything out of the pictures it is sending back - but as its surroundings clear they finally see what they have been looking for: a great ice cliff stretching above it. Their probe has reached one of the most important points on the planet.

The submarine, Icefin, is part of a joint British-American project to understand what is happening to the Thwaites GlacierA mass of very slowly moving snow and ice. Experts believe that at their fullest extent, during the Ice Age, glaciers covered 30% of the Earth's surface.. Scheduled to last five years, and costing $50 million, the study is being carried out in immensely challenging conditions, part of it in an area never previously visited.

The Thwaites Glacier is vast: covering 74,000 square miles, it is approximately the size of the UK. Its front is nearly 100 miles wide but is collapsing into the sea at a rate of about two miles a year. During the 1990s it lost just over 10 billion tonnes of ice per year; this has now risen to 80 billion.

This melting currently causes about 4% of the world's rise in sea level - and experts warn that it is on course to have a far greater effect. "The big question is how quickly [the glacier] becomes unstable: it seems to be teetering at the edge," says Paul Cutler, programme director for Antarctic glaciology at the USA's National Science Foundation.

If the whole glacier collapsed, sea levels would rise by more than half a metre. But the Thwaites, Cutler says, is not just a worry on its own account. "It is a keystoneIn architecture, the keystone is the central stone at the top of an arch that locks the whole structure together. for the other glaciers around it in West AntarcticaThe most southerly continent, surrounding the South Pole. Its area is estimated at 5.5 million square miles, most of it covered in ice.. If you remove it, other ice will potentially start draining into the ocean too." That could add another three metres to sea levels.

The ice in Antarctica - some of it nearly three miles thick - holds around 90% of the world's fresh water. The glaciers in the east of the continent are situated on high ground and only slipping into the sea very slowly. But in West Antarctica, most of the ice is below sea level, and much more vulnerableat risk of being harmed.

The ice is being melted by seawater from the other side of the world, carried by a deep ocean current called the Atlantic Conveyor down to Antarctica. There, it becomes part of the Antarctic Circumpolar Current, which travels a third of a mile below the freezing surface water. Although it remains extremely cold, it is still warm enough to melt the ice it comes into contact with. And it is reaching more of the Thwaites Glacier, scientists say, because of stronger winds on the edge of West Antarctica - ultimately caused by the warming of the Pacific Ocean.

The pictures of the ice cliff sent by the Icefin submarine are vital because they show where the warmer water is meeting the front of the glacier. Researchers have now established that warm water is reaching the underside of the glacier through a series of deep-water channels - some of them 600 metres deep - which join up to create an enormous cavity inside it.

Some, including climate scientist Anders Levermann, believe not. They say that even if the glacier melted completely, it would take decades. "If you're not stupid," Levermann says, "you don't die of sea level rise because it's so slow that there's time to protect against it or abandon the land. Having said that, being stupid would include not taking climate change and sea level rise seriously."

Others argue that the disappearance of the glacier would be disastrous. Since 1993 sea levels have risen by 91 millimetres, and even that relatively small amount has caused severe coastal flooding - which we have failed to cope with effectively. The melting of the Thwaites Glacier would cause problems on a far larger scale, and we seem to have neither the will nor the resources to deal with these.