10 Most Annoying Unanswered Questions In Star Trek

2. Carl

The Guardian Of Forever remains a mystery, though Terra Firma did go a little bit further towards granting us clarity. First introduced in The City On The Edge Of Forever, it returned in Yesteryear and was originally pitched to return in what would become Yesterday's Enterprise. As Carl, the interactive and humanoid interface, it made its final (to-date) appearance in Terra Firma Part 2.

Carl tells the audience that he moved himself to Dannus V to avoid the abuse of his power by combatants in the Temporal Wars. However, this same conversation includes another revelation - at least in this guise, he can send people across dimensions, as well as through time. This was alluded to by Spock centuries prior, but only then fully realised.

What, then, is he? He claims that he is 'his own beginning and his own end,' but also claims that he was created to show history in a certain way. Has this changed over the centuries? Spock surmised he was at least '10,000 centuries old,' which seems a far longer time to hone a craft than the scant millennium between his first and most recent appearance. 

The Guardian was a fantastic invention by Harlan Ellison - we'd just like to know a lot more about it.