Except for the Rishi Ganga gorge to the west, Nanda Devi is encircled by a ring of mountains with no col lower than . This mountain chain surrounds and completely encloses the Nanda Devi Sanctuary. Tom Longstaff had reached an eastern col in 1905 and had been able to see into the Sanctuary but he did not try the formidable descent. The immensely deep and narrow gorge by which the Rishi Ganga river drains the Sanctuary had never been ascended despite many attempts. However, before the 1934 monsoon, Shipton and Tilman along with three Sherpas who they regarded as co-climbers - Ang Tharkay, Pasang and Kusang - became the first people to find a way through the gorge and to set foot in the Sanctuary in what has been described as the most exciting story in the whole saga of mountain discovery.