In the mid-19th century, Tibet closed its frontiers with India. and British Indian officials, caught up in the Great Game and fearing Russia's invasion of India, desperately wanted intelligence on their northern neighbor. On their maps, Tibet was one huge white blank. But travelling into Tibet was treacherous for Europeans: murderous tribes, armed bandits, extreme terrain and weather, and border guards who waited at every pass.

It would require foolproof disguises, deception and secrecy. The ingenious technique devised by British surveyers was to train the Pundits to take precise footpaces (thirty-three inches exactly in the case of Nain Singh, see below), use a Buddhist rosary to keep exact count of the paces in a day, or between two landmarks, and use prayer-wheels fitted to hide their notes and conceal compasses.

This is an account of Nain Singh's third and final secret penetration of Tibet. He was now known to border officials and needed extra precautions to ensure his disguise. From July to November 1874, pacing his route, he covered 1,095 miles from Leh to Lhasa - then turned south and mapped an unknown part of the great Tsangpo River, crossed the main Himalayan chain at a16,000 foot pass, and took a route through Tawang to British India, ending in Udaiguri on March 1, 1875. In disguise, and mapping virtually unknown country, Nain Singh had covered 1,405 miles.