A new book has described how the US collaborated with India in the 1960s to place nuclear-powered monitoring devices across the Himalayas in order to spy on Chinese nuclear tests.
China detonated its first nuclear device in 1964.
The book by one of India's most famous mountaineers, Captain Mohan Singh Kohli, Spies in the Himalayas, says the programme was kept a secret for 38 years.
Captain Kohli - who was part of the spying programme - also writes about the famous 1970s sailing expedition on the Ganges, led by Sir Edmund Hillary.
Amongst other things it was intended to monitor the river for any nuclear contamination from a missing monitoring device that was lost atop the Nanda Devi mountain.
From the newsroom of the BBC World Service