The mountains can be incredibly hostile and dangerous if you are left to fend for yourself overnight. We leave Graham to survive a night alone in the wilds of the Andes - while Ed leads a rescue party to find him the next day.
Ed then travels with historian Dan Snow to Finland to tell the incredible story of the 1939 Winter War, when the country was invaded by Russia.
They will explain some of the innovative tactics used - and experience first hand some of the modern training methods that make the Finns such a formidable winter warfare nation.
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Ready to Watch High Altitude, Season 1, Episode 4?