The ascent of Nanda Devi necessitated fifty years of arduous exploration in search of a passage into the Sanctuary. The outlet is the Rishi Gorge, a deep, narrow canyon which is very difficult to traverse safely, and is the biggest hindrance to entering the Sanctuary; any other route involves difficult passes, the lowest of which is . Hugh Ruttledge attempted to reach the peak three times in the 1930s and failed each time. In a letter to "The Times" he wrote that 'Nanda Devi imposes on her votaries an admission test as yet beyond their skill and endurance', adding that gaining entry to the Nanda Devi Sanctuary alone was more difficult than reaching the North Pole.[http://books.guardian.co.uk/reviews/travel/0,6121,1201876,00.html]