七五八大洪水 (literally, 75-8 Great Flood / qiwu-ba dahongshui) is a reference to the massive rainfall and subsequent flooding that took place in Henan province in August of 1975.
Why it is blocked: During that event, the Banqiao Dam collapsed, with estimates of 90,000-230,000 people dying. Reports are that poor construction, combined with a record rainfall, were to blame. However, files about the incident have already been declassified for an event that occurred over 35 years ago, so it’s curious why this is still blocked.
