"In August 1975, the Banqiao Dam and 61 others throughout Henan, China, collapsed following the landfall of Typhoon Nina. The dam collapse created the third-deadliest flood in history which affected 12,000 km2 (3 million acres) with a total population of 10.15 million, including around 30 cities and counties, with estimates of the death toll ranging from 26,000 to 240,000."

https://en.wikipedia.org/wiki/1975_Banqiao_Dam_failure

"After the disaster, the Chinese Communist Party and the Chinese government remained silent to the public, while no media were allowed to make reports."

"The official documents of this disaster were considered a state secret until 2005 when they were declassified."