So, while I was up at FOGcon* over the weekend, I popped into the local Whole Foods for some fruit, and was greeted by a big stack of citrus fruit labelled as "sumo mandarins". They were billed as "the sweetest orange you'll ever eat!" So I bought a couple.
They're big (a bit bigger than my fist) and look a bit like a tangelo that's been partying late at FOGcon and is looking a bit rumpled and wan. Photo below cut:
the sweetest oranges you'll ever taste, at least in terms of a lack of acidity. I actually find the lack of tartness a bit disconcerting, but there's also an interesting note to the flavor that I can't quite pinpoint, but is quite nice.
I have no idea if this is just a local thing that somebody is test marketing, or if they are generally available, but if you spot them, I recommend giving them a try.
*FOGcon, by the way, was fantastic, and I will try to post some kind of proper summary as soon as my brain recovers.