Islands of foam have linked up on the surface, bubbling at rate of one every 1-2 seconds. The formation of foam itself appears to be cyclic; 30 minutes after writing the last sentence I happened to notice that the irregular quarter inch deep foam on the surface had receded back to large, shallow islands again. A further 20 minutes later and we’re back to a solid layer of foam, approaching an eighth of an inch in thickness here and there. Weird.