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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 - and already accounts for 4% of world sea level rise each year. But satellite data shows that it is melting increasingly rapidly.

For the first time this year, a large-scale scientific survey is happening on the glacier. And Chief Environment Correspondent Justin Rowlatt was invited along.

(Photo: Aerial view of Thwaites tongue from high above the Amundsen Sea in 2016. John Sonntag / NASA)