I don't think there have been very many (if any) human achievements were achieved by people that thought they were impossible/uncontrollable. Very difficult,almost impossible sure, but not impossible.

To give an example: People have been working on nuclear fusion for many decades without any real success yet. So it's surely one of the, if not the hardest problem humans have ever worked on. However, I highly dought anyone working on fusion genuinely thinks it's impossible/uncontrollable. Just very very hard.