ANTARTICA-Sediment sweeps past the camera as Icefin, a bright yellow remotely operated robot submarine, moves tentatively forward under the ice. Then the waters begin to clear.

Icefin is under almost half a mile (600m) of ice, at the front of one the fastest-changing large glaciers in the world. Suddenly a shadow looms above, an overhanging cliff of dirt-encrusted ice.

It doesn’t look like much, but this is a unique image - the first ever pictures from a frontier that is changing our world.

Glaciologists have described Thwaites as the “most important” glacier in the world, the “riskiest” glacier, even the “doomsday” glacier. It is massive - roughly the size of Britain.

It already accounts for 4% of world sea level rise each year - a huge figure for a single glacier - and satellite data show that it is melting increasingly rapidly.

There is enough water locked up in it to raise world sea level by more than half a metre. And Thwaites sits like a keystone right in the centre of the West Antarctic Ice Sheet - a vast basin of ice that contains more than 3m of additional potential sea level rise. Yet, until this year, no-one has attempted a large-scale scientific survey on the glacier.

The Icefin team, along with 40 or so other scientists, are part of the International Thwaites Glacier Collaboration, a five-year, $50m (£38m) joint UK-US effort to understand why it is changing so rapidly.

The project represents the biggest and most complex scientific field programme in Antarctic history. It takes the science teams weeks just to get to their field camps. At one stage, the entire season’s research is on the point of being cancelled because storms stop all flights to West Antarctica from McMurdo for 17 consecutive days.

Why is Thwaites important?

West Antarctica is the stormiest part of the world’s stormiest continent. And Thwaites is remote even by Antarctic standards, more than 1,000 miles (1,600km) from the nearest research station.

Only four people have ever been on the front of the glacier before and they were the advance party for this year’s work. But understanding what is happening here is essential for scientists to be able to predict future sea level rise accurately.

The ice in Antarctica holds 90% of the world’s fresh water, and 80% of that ice is in the eastern part of the continent.

The ice in East Antarctica is thick - more than a mile thick on average - but it rests on high ground and only creeps sluggishly to the sea.

Some of it has been around for millions of years. Western Antarctica, however, is very different. It is smaller but still huge, and is much more vulnerable to change. Unlike the east it doesn’t rest on high ground. In fact, virtually the whole bed is way below sea level. If it weren’t for the ice, it would be deep ocean with a few islands. I’ve been in Antarctica five weeks before I finally board the red British Antarctic Survey Twin Otter that takes me to the front of the glacier.

I will be camping with the team at what is known as the grounding zone. They are camped on the ice above the point where the glacier meets the ocean water, and have the most ambitious task of all. They want to drill down through almost half a mile of ice right at the point where the glacier goes afloat. No-one has ever done that on a glacier this big and dynamic.

They will use the hole to get access to the sea water that is melting the glacier to find out where it is from and why it is attacking the glacier so vigorously.