Hispar Glacier is a 49 km (30 mi) long glacier in the Karakoram Mountains of the Northern Areas, Pakistan which meets the 63 km (39 mi) long Biafo Glacier at the Hispar La (Pass) at an altitude of 5,128 m (16,824 ft) to create the world's longest glacial system outside of the polar regions. This 100 km (62 mi) highway of ice connects two ancient mountain kingdoms, Nagar (immediately south of Hunza) in the west with Baltistan in the east. The extreme steepness of the hillsides and strenuous nature of the boulder hopping on the lateral moraines and hillsides make this route's upper half the most difficult part of the Biafo - Hispar traverse. Only the Hispar La day includes walking on the Hispar Glacier. The crossing of four major tributary glaciers from the north is most taxing, and potentially high nullah crossings can be dangerous. The views of 7,800-metre (25,600 ft) peaks and of the snow covered cliffs and mountains on the south side of the glacier are particularly impressive.