The deepest valley in the world.And the valley is rich in biodiversity In eastern Nepal, the Arun Valley cuts a deep gorge through the heart of the Himalaya. In the remote high mountain region of its tributary, the Barun, glaciers spill off mighty Makalu, the fifth highest peak in the world. It is famous for the trackking to Makalu Barun Conservation area , Arun River is in places densely forested, contrasting with the terraced slopes higher up, which are farmed by villagers of diverse tribal groups. High pastures typify the upper slopes, which border on the Makalu-Barun National Park, an important conservation area for many threatened species.