In 1874 a native explorer, the pundit Nain Singh, started on his famous journey from Leh to Lhasa, and was instructed, if possible, to follow the Tsangpo and see where it went. He reached Lhasa, and on his return struck the Tsangpo at Tsetang, well to the east of the point where the British expedition crossed in 1904. He followed its course for thirty miles farther down, but was prevented from continuing his journey and compelled to return by the direct route to India. In 1878 another native explorer, G.M.N., seems to have followed the Tsangpo down as far as Gyala, which is not far from the point where the river turns sharply to the south, but his reports were not considered reliable. In 1884 another native, Kinthup, succeeded in following the Tsangpo to a point called Pemakochung. There he found an enormous gorge, and was compelled to make a detour out to the north and east, rejoining the stream where it entered the Abor country. Kinthup's report was of the highest interest. He had stood at the beginning of an apparently impassable gorge, and he reported a fall at Pemakochung of 150 feet. He was, however, quite illiterate and was only able to make his report from memory, and it presently appeared that the height might be only 50 feet, and that the higher fall was not in the main stream but in a small tributary. One fact, however, of the utmost importance had been established by his expedition. The Tsangpo was beyond reasonable doubt the Brahmaputra in its upper course. [Illustration: The Gorges of the Brahmaputra.] The Lhasa expedition in 1904 would fain have traced the river to the plains had not the Government interposed a veto. In the years that followed, the source of the Tsangpo was discovered by Captain Rawling. In 1911 the Abor expedition increased our knowledge of the course of the Brahmaputra right up to the skirts of the main range. The problem now was not the linking up of the Tsangpo and the Brahmaputra, but what happened to the river in the hairpin bend between Pemakochung and its debouchment in the Abor valleys. The elevation of the stream at the point where the main road to Lhasa crossed it was in the neighbourhood of 12,000 feet. From there as far as Pemakochung we knew that there was no very great loss in altitude, but when the Brahmaputra appeared in the Abor foothills it was only between 1,000 and 2,000 feet above the sea. The stretch of unknown course was perhaps 200 miles, and in that section the river broke through the main range of the Himalaya. It was possible--nay, it was probable--that somewhere in those gorges, which Kinthup had thought impassable, lay hidden the most tremendous waterfall in the world. The secret of the Brahmaputra gorges was one of the topics that most fascinated geographers between the years 1904 and 1913. In that latter year the mystery was solved, and the _ignotum_ proved not to be the _magnificum_. This is the story of the solution.