In his famous Lectures on Physics, some of the lectures repeated in the 1967 Messenger Lectures at Cornell and published as The Character of Physical Law, Feynman famously said that “nobody understands quantum mechanics” and that the two-slit experiment contains the “one mystery” of quantum mechanics.
“I will take just this one experiment, which has been designed to contain all of the mystery of quantum mechanics, to put you up against the paradoxes and mysteries and peculiarities of nature one hundred per cent. Any other situation in quantum mechanics, it turns out, can always be explained by saying, ‘You remember the case of the experiment with the two holes? It’s the same thing’. I am going to tell you about the experiment with the two holes. It does contain the general mystery; I am avoiding nothing; I am baring nature in her most elegant and difficult form.”