We're not sure if Guinness bubbles behave differently underwater, but at least on land, while most bubbles from beer rise to the top of the glass, Guinness bubbles fall. Finally, scientists (from Ireland, of course) have figured out why Guinness bubbles act differently than their other beer cohorts.
The reason: the glass. The shape of the glass determines how a Guinness or stout beer is poured in, shaping the bubbles. Previously, the researchers said to the BBC that it was thought the "circulatory current" of the beer created the bubbles. But when the scientists experimented by pouring a stout into a regular pint glass, the bubbles went up instead of down. When a Guinness is poured into a stout glass — with a narrower bottom — the flow "is directed towards the wall and upward," as the sinking liquid drives rising bubbles down.
Plus, stout beers contain nitrogen, instead of carbon dioxide like most lagers. It creates different-sized bubbles; bubbles in lagers are larger and aren't as prone to that drag down, the researcher said to the BBC.
You can see for yourself at home: the researchers devised an easy at-home experiment. Pour your next Guiness in a cylinder at a tilt, and you'll see bubbles will move both up towards the top and down.