@skyfaller I'm certainly not looking to start a fight, just a conversation.
That's an interesting article, haven't seen LTM before, but at least in my mind this is distinct from the other examples being discussed. We could absolutely still build these things if we wanted to - the knowledge is there, it's not entirely lost to society.
I'm more looking for knowledge that has been entirely lost. Which, I guess, is impossible to prove, since, well, we lost it? I dunno!