If the Holocaust didn't happen, what did, exactly?

Some people have questioned the claims of the Holocaust or denied that it happened at all. (NOT ME.) If the Holocaust is a myth, I’d like to hear what exactly you think did happen. What was going on at the concentration camps? Or are the concentration camps all fake? Are the hundreds (thousands) of testimonials a form of mass hysteria? Or coordinated lying?

It’s very easy to simply question the claims of accepted history. But it’s more difficult to present alternate claims that actually make sense in light of the evidence. Holocaust-deniers—here’s your big chance!