Big Secrets Kept for Decades:National Security Agency (NSA) and Widespread Domestic Spying
NSA is established in 1955 but domestic spying had already begun in 1945. 1st made public in congressional hearings on Domestic Spying 1975.Recent Snowden revelations regarding widespread  domestic spying via internet traffic capture - 2013.Kept secret 30 yearsArea 51  Experimental Airforce Testing Ground - EST. 1955
Bob Lazar story of working on back engineering captured E.T. Craft at S4 - 1989. 1st  Official US disclosure of Groom lake base - 1994Kept Secret 34 yearsNational Reconnaissance Office - EST. 1961
Operators of space based radar and surveillance
1st made public - 1992Kept Secret 31 yearsAmnesty for Japanese Biological and Chemical Warfare Unit 731 - EST. in 1932 Human experimentation program using civilian and POW’s running until 1947. The date that a single US official acknowledges that amnesty was granted to Japanese war criminals in exchange for data on this brutal experimentation - 1998.Kept Secret 51 yearsOperation Paperclip - EST 1945US Military imports 1600 German nationals and Nazi Scientists (Many who were proven war criminals) & grants them amnesty in the US leading to the creation of NASA and US Missile Weapon operations. Finally officially acknowledged in 1998.Kept Secret for 53 years