Making the largest valley in the world, the Arun Valley, this himalayan mass was considered the highest mountain in the world, before other higher mountains were discovered. This mountain forms a base to the valley as well as to the Makalu-Baun Conservation Area. This mountain gives birth to the Arun river, one of the largest in the country and a pilgrimage to Arun-Barun Dovan. Arun valley is the largest collection of birds (over 650 species of birds) and butterflies (over 800 species of butterflies), and shelters the rare and endangered red Panda. The double pass enroute to the base camp is considered difficult and dangerous, because one leads to the Tibetan pamirs from where the trails disappear, and the other to the Base Camp.