It's amazing what's been covered up.
- The Tiananmen Square demonstrations. (1989)[1] At least in China.
- Britain's efforts to get the US into WWII before Pearl Harbor. (early 1940s) [2]
- DuPont's attempt at a US coup (1933) [3]
- The Israeli nuclear bomb test. (1979) [4]
The first one is a successful outright suppression. The rest were obfuscated and obscured at the time, but the historical record exists.
[1] https://en.wikipedia.org/wiki/1989_Tiananmen_Square_protests...
[2] https://www.history.com/news/wwii-us-entry-secret-british-ca...