My favourite? A small rivulet called the Yarlung Tsangpo, which starts in the vicinity of the Manasarovar lake, traversing the Tibetan plateau to the east, going past Lhasa… Due to the obstruction created by Namcha Barwa and Gyala Peri, both high mountains above 7,500m, the Tsangpo abruptly changes its course to form a deep and frightening gorge to turn south. From 4,800m, it falls rapidly to 680m to enter Arunachal Pradesh. Now known as the Siang, it reaches the plains of Assam to be called the Brahmaputra. It took almost 150 years to explore and to chart the course of this river.

Famous Pandit explorer Kinthup reached within 80km of the entry point into India from the north (Tibet). From the south (Arunachal), the British could not penetrate deep into the hostile tribal territories. After the Indo-China war, the area was out of bounds. Though the course of the river was charted by modern methods, the bend guarded by thick jungles and almost impassable terrain, was never reached from the south. Charles Allen called it “the last great Asian adventure — a journey all the way up the Tsangpo-Brahmaputra from the Assam valley to the Tibetan plateau.” This was the exploration that we did.

Following the Siang we reached the Guyor La, a high pass on the border, which offered a view of Namcha Barwa. It was six days of hard work, blazing a trail through an overgrown forest and trekking on nonexistent trail, where local porters were of great support. Surprisingly in this evergreen forest, water was difficult to obtain. In the next two days, we descended steeply, almost gliding down to the Siang. The night on the bank of the fast flowing Siang was magical; the velocity and roar of the gushing river was both fearsome and invigorating. Next day, we reached the point where the river flowing in from Tibet makes a huge S-bend to enter India. Standing here and completing a historic exploration was the elixir of my trekking life.