There is something called an authenticated append only skip list, which I think is kinda neat. It is basically what it sounds like. Iirc it allows certificates

There is something called an authenticated append only skip list, which I think is kinda neat. It is basically what it sounds like. Iirc it allows certificates showing that something has a particular place in the list, given the assumption of the latest thing added to it, in length at most logarithmic in the length of the list.