Gerencher's Guinness Page
My interest in Guinness, although originally motivated by taste and social context, also involves some degree of scientific curiosity. On many occasions I have noticed that bubbles in a freshly poured Guinness can be observed to descend, rather than ascend. Based upon my growing experience with beers, I find this unusual property to be almost unique to Guinness, and is most pronounced when Guinness is poured into tapered pint glasses, rather than straight-sided ones. The only other beer that I have encountered that exhibits this property is Boddingtons Ale, which, incidentally, is not a dark beer.
For an explanation of this phenomenon, see the article by Clive Fletcher in the January 14, 2000, issue of Science, volume 287, page 221, entitled "The Unbouyant Bubbles of Guinness."