Nuckelchen has posted this TBS/JNN video from Sept. 4, showing black smoke coming up out of the ground between reactors 2 and 3. I believe this is due to a criticality in underground corium #2, the big one. Smoke has poured out of the ground at this location before. Note that it is black, and there is a black substance covering a good part of Japan now, with extremely high radiation, and especially alpha radiation. It is likely that the corium itself is being aerosolized.
Presumably the corium has encountered groundwater. Water moderates the nuclear reaction, slows neutrons down enough to generate a re-criticality. This is becoming more frequent, it appears to be happening almost daily now.
Looks like some uranium- and plutonium-heavy plumes are headed for the Pacific northwest this coming weekend. These are probably going to be the long, thin stringy plumes that hit a few places hard, and it’s not so bad away from them.
UPDATE: Mochizuki from Fukushima Diary reports krypton-85 from unit 1, ongoing fission.
So it looks like both coriums #1 and #2 are acting up at the same time.