Movies whose original elements are "lost"

[Bolty]

I wonder why certain movies don't have proper modern restorations. For some perhaps the "owners" don't feel that there is a payoff in restoring the movie -- of course I think of Star Treks 1, 3-10 where Paramount almost certainly has the original elements but won't cough up the money to restore them.

But other movies seem to have all or parts missing for a "perfect" restoration. The Thing (1951) supposedly is missing some of the original 35mm elements preventing a full restoration and they seem completely unwilling to either release the restored shorter cut or do a hybrid cut mixing the sub-par portions with the better elements.

Two of my favorites -- Forbidden Planet and Logan's Run have fairly average transfers for their latest releases and I have heard that the original elements for those two are lost. (I have no idea if that is fact.)

Clearly, "It's a Mad, Mad, Mad, Mad World" -- had some elements missing and they did choose to make a hybrid cut.

So what are some other movies where the original elements are lost and we'll never get a proper Blu-ray -- let alone UHD of?