Tsundu Tsangpo (1385-1464), more commonly known as Thang-stong rGyal-po, "The Builder of Iron Bridges", is famed throughout the Tibetan Buddhist world as a great yogin, sage and engineer. Among his most famous teachings is his sadhana for Avalokiteshvara. The prophecies he received from Avalokiteshvara became the major impetus for his extensive bridge construction, an endeavour seen as part of the Bodhisattva vow to help all sentient beings. He is thus credited with building most historic bridges in Tibet, in particular the horizontal cantilever bridge spanning 450 ft. across the Brahmaputra river at Phomi from non-corroding iron alloy circa 1420. The bridges were built by collecting iron on orders from various chieftains, or freely donated by the communities, wishing to facilitate transportation between one place and another. During his long and active life he traveled all over Tibet as well as to India, Kashmir, Ladakh, Mongolia, China and Bhutan, teaching and spreading the Dharma. He is also connected with the origin of the Tibetan drama tradition and the monastic orchestra; cf. J. Needham, Science and Civilisation in China, 1954-71, vol. IV, part 3, p. 206; G. Tucci, Tibetan Painted Scrolls, 1949, vol. 1, p. 163; and J. Gyatso, The Teachings of Thang-stong rGyal-po, in: M. Aris (ed.), Tibetan Studies in Honour of Hugh Richardson, 1979, p. 111ff.
The inscription may be translated as follows: "Praise to the Guru! Just by your glance, you subdue all sentient beings. Just by mere threat, you enslave the eight demonic forces. Just by mere thought, like the falling rain your desires are abundantly fulfilled. We bow to the great saint Tsundu Tsangpo."