Scottish scientists claim to have video evidence which proves that bubbles in a glass of beer can move downwards.
Dark beers were reported to produce the best results for researchers
Working in association with colleagues in California, the Edinburgh University researchers used a high-speed digital camera to record the movement.
Until now, some had claimed that the effect was an optical illusion.
The scientists released their evidence to coincide with St Patrick's Day, claiming that the effect is clearest in Irish stout.
Although generations of beer drinkers had reported seeing bubbles sink to the bottom of their glass, scientists had struggled to prove that it could really happen.
Conventional wisdom suggests that bubbles should float to the surface.
Recently Australian researchers used industrial simulation methods to demonstrate that bubbles would sink when a pint is poured.
Those behind the joint Edinburgh University and Stanford University project said their study had given definitive proof.
The researchers found that bubbles touching the walls of a glass experience drag which prevents them floating upwards.
Bubbles in the centre of the glass are free to rise, creating a circular flow which causes the bubbles near the edge of the glass to be pushed downwards.
The effect occurs in any liquid, but the scientists claim that the contrast between dark liquid and creamy bubbles in stout made it most suitable for their research.
Dr Andrew Alexander of Edinburgh University's School of Chemistry said: "We had known for some time about the question of Guinness bubbles defying the norm.
"Our group carried out preliminary experiments at a local pub a few years ago, but the results proved inconclusive.
"But now we have produced video proof that the bubbles do actually go down the inside of the glass."
Although an undisclosed quantity of beer was used in the laboratory experiments, the researchers insist that it was poured away afterwards to prevent their judgement becoming clouded.