They form part of a unique team in the British Armed Forces.
Each year a select handful of personnel see if they've got what it takes to become a Royal Marines Mountain Leader. 
There's only 100 of the mountain and cold-weather warfare specialists currently serving in the branch, which has its roots in World War Two. 
The Mountain Leaders 'Two's Course', which can take up to nine-months, is one of the toughest in the business. Sgt Jonny Garbutt, the course's Sergeant Major, said:
"It's the intensity from day one of the course all the way through to the end of the Norway phase... It really doesn't ease up. People don't always have a background in climbing, skiing, surveillance or reconnaissance."
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