On the night of 12 November 1970, a tropical storm (known as a Cyclone in the Indian Ocean) that had been building to the south for some days, made landfall near the border of West Bengal (India
) with East Pakistan (now Bangladesh).
The effects were made worse by a tremendous storm surge driven by it from the Bay of Bengal and the funnelling effect of the coastline. Estimates of the casualties vary but up to half a million may have lost their lives and countless others were flooded out. Thousands of hectares of food crops were also destroyed.
Many services sent aid to relieve the victims of the Bhola Cyclone and ships from the Royal Navy and the Royal Fleet Auxiliary were amongst the first on the scene. The role of HMS Hydra was primarily to re-chart the sea floor around the Ganges delta (after the 4 metre tidal wave had swept though it), assisted by recent satellite images from the US Navy
, to ensure safe passage and anchorage for other relief vessels.