Partially true, really? Roswell, chem trails, fake moon landings, 9/11 and the third building... I could go on but I think the point is made.

@Denker despite all evidence to the contrary, conspiracy theorists cling to their belief.......hmmm sounds like what creationists do

It does not need to be kept secret by many people, just one or two in power then a lot of others who find the lie so appealing or are so slavishly devoted to those who know it is a lie that that they will never question it.

@LenHazell53 You mean like the moon landings? 15 astronauts (18 if you count Apollo 13). numerous technical staff, both at NASA and in Australia where they relayed the signals. Not one of them got drunk and told their wives gfs, or bestie.
9/11 is a classic example of denial by conspiracy. The most powerful nation in the world cannot accept the fact that it let 6,000 of its people be killed by some nutters with craft knives. So it looks for reasons other than its own inadequacy. "A worldwide zionist plot to which our own govt agencies were complicit".
Lady Diana, all those people who would buy magazines with her picture on were the people who killed her. If no one gave a shit then the car would not have been going so fast. But "oh no it can't be our fault. Lets blame the royal family"
I could go on but whats the point. I am just an alien robot put here to make you all feel safe whist we drain your blood and poison you with fluoride.

@273kelvin I did not cite those examples you did, I was simply making a point that if someone in power wants to cover something up, they can and do with only the very minimum of confidants, J E Hoover is a prime example, his staff did what he told them without question including spying on ordinary citizens, celebrities and politicians, when he died it was found he had thousands of personal hidden files on thousands of people, many of whom he had been blackmailing for years to stay in his job and to get dirt on others, to this day it is thought doubtful anyone else was directly involved and if they were we don't know who they were.
You only have to look at the corruption of the Catholic church to see how actual conspiracies can be covered up for decades or even centuries The Magdalene laundries for example, even the church itself "claims" to have known nothing about the abuse, mistreatment and even murders carried on inside those wall and blames it on a conspiracy of Nuns.
There dozens of conspiracies that have proven to be right or true in the fullness of time, the mega conspiracies you mention are the big lies, meant to distract from the small truths that governments, religious authorities and institutions geared for the benefit of the very rich want tarred with the same brush.