On August 4, 1928, Underhill, accompanied by Miriam O'Brien and guides Armand Charlet and G. Cachat, completed the first ascent of the traverse from the Aiguilles du Diable to Mont Blanc du Tacul in the Alps. This route involves climbing five outstanding summits over 4000 meters in superb surroundings. On this same trip, Underhill completed guided ascents of the Peuterey and Brenva ridges of Mont Blanc. His climbing partner Miriam O'Brien was later to become his wife, and a famous mountaineer in her own right.