IIRC it came to light when documents that were supposed to be shredded were declassified by mistake. The government has shown that it's actually really good at keeping secrets, or at least keeping things quiet. Elsewhere in this thread there's the discussion of a possible FBI program to get people to commit mass shootings that's widely considered to be a crazy conspiracy theory despite there being individual documented examples ie https://www.cnn.com/2017/11/29/politics/aby-rayyan-fbi-terro...

My whole thing with MK-ULTRA and Ted is that researchers were happy to do stuff as damaging and way less (potentially) useful than his experiment.

I disagree.

Let's say I work on a project where we keep Elvis alive in a super secret bunker in antarctica. What can I do? Call the cnn and tell them? Make a post on 4chan? Make such a claim here? Look, I'm leaking top secret info, and nothing will happen. Until CIA officially admits keeping Elvis there, noone will believe me... (except a few conspiracy theorists, branded as nutjobs by the general public).

If I know that an entity goes to great lengths to conceal secrets, why would I ever believe I know of their most secret secret?

How do you know it was their most secret experiment?