He's trekked around Snowdonia in North Wales and explored the Yorkshire Dales - now Bear Grylls goes north of the border to the Scottish Highlands on an adventure that takes him from the deepest point in Britain to its highest mountain, Ben Nevis. Along the way he goes in search of freshwater pearl mussels, one of the most endangered species in the world, heads for the hills to discover how the country's magnificent highlands were formed and hears about a plan to save an ancient forest under threat because the native Scots pine has been over-run by a non-native species. He also discovers how reindeer are making a comeback and heads up the formidable Aonach Mor, a mountain more than 4,000ft tall with treacherous steep ice walls.