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A year ago, we were swimming in the sea and there were divers below us.

It was quite fun to look at the very large bubbles of air coming up around us.

Then, a question popped up: assuming a stream of large bubbles coming from below you, could you be dragged down?

My interpretation is that the bubble in contact with you wouldn’t offer the same support as water, so you would go slightly deeper and by doing so you’d have a bit of inertia as well, but then comes the next bubble.

So with the right size and stream of air bubbles, could one go down?

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  • $\begingroup$ I think the answer is "yes", but it would be a fun experiment to set up to get a definitive answer. Perhaps a large, tall transparent graduated cylinder with a tube blowing air in at the bottom, and some neutrally buoyant objects suspended in water in the cylinder. Use a video camera to record what happens. $\endgroup$
    – S. McGrew
    Feb 7 '20 at 14:50
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    $\begingroup$ youtube.com/watch?v=C7A645F-3sk $\endgroup$
    – BowlOfRed
    Feb 7 '20 at 16:59
  • $\begingroup$ It is a known effect. When we visited the Niagara Fall we saw warnings about reduced buoyancy in the water below the fall where large amount of air bubble is mixed into the water. $\endgroup$
    – verdelite
    Feb 7 '20 at 19:45
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One would have to give the density of bubbles/water. One floats because of buoyancy, and people float in the sea because salt water is denser than the average human body. If air bubbles replace water, the buoyancy will drop and certainly the body will no longer float on top. How deep it will go needs specific density numbers.

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A similar effect has been hypothesised to be behind the loss of ships at sea: "Lab tests have proved that bubbles can sink floating objects. The findings add weight to suggestions that methane bubbles escaping from methane reserves in the seabed might have been to blame for vessels disappearing in the Bermuda Triangle and the North Sea." from New Scientist.

New Scientist articles can be a little sensationalist, but the study they report here from Naval Postgraduate School in Monterey, seems to be reasonably convincing.

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