Originally published at: https://boingboing.net/2020/11/25/snails-can-crawl-on-the-underside-of-water-surfaces.html
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But what is the snail getting out of this? Is it just for funsies?
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Key question: How long can they hold their breath for?
@bobtato
To get to some surface that will let them get out of the water.
I regularly find snails on the insides or edges of various water containers - e.g. watering cans. They go in looking for somewhere nice to hole up and then get flooded and then seek to get out.
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Can’t speak for this individual snail, but aquatic snails often eat the bacterial film that grows on the water surface.
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There are lots of species of aquatic snails that live their whole lives in water. Some snails have pallial lungs and others have gills. For the critter in the video, the answer may well be that it breathes under water and doesn’t hold its breath at all.
ETA: If the guess of snail genus in the top video description is right, then it can’t breathe under water and would drown if it couldn’t find a surface to flip onto
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Seems like if you’ve devoted considerable resources to growing a shell over your top half, flipping over and exposing your soft underbelly might not be a winning move, evolutionarily speaking
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The ones in my watering cans and other containers drown if stuck in the water for too long. I have a large tub catching the overflow from one of my water butts and from time to time I take the cover off and see handfuls of drowned snails.
I’m not sure why aquatic snails would need to crawl on the under surface of the water using water tension. They’d just do whatever passes for snail swimming, wouldn’t they?
In the first video in the post it looked like a common or garden snail that swum to the piece of greenery and used it to get its head out of the water.
ETA - ah - just saw your ETA. 
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Because they can! Evolution patiently waited 1.63M years for youtube to come into existence to show this off, don’t spoil it for them now!
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When there is a rapid change in barometric pressure, aquatic snails can find themselves floating, which is bad news. They become vulnerable to predatory fish. I would presume they are looking for shoreline or emergent vegetation to escape back down the water column.
I’ve caught trout that have obviously been gorging on snails. It’s one of those mystery hatches where you see rising trout but can’t figure out what they are eating. Eventually, you find a snail upside down under the surface film and it clicks.
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Also seems like losing your shell completely might not be a winning evolutionary move, and yet that’s how we got slugs.
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Completely non scientific and anecdotal, but we’ve noticed a major shift in the balance of slugs to snails in our yard over the past 5 years. Snails were a rare sighting, and we had a major surplus of slugs, including our beloved PNW giant banana slug. Now, we hardly find slugs and when we do, they are tiny. Meanwhile, snails have taken over.
Climate change? Drought?
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Snails can survive drought by closing up their shell with an epiphragm, so if there was a dry period they’d be better able to survive than slugs.
There are lots of other possible reasons though. My wild guess would be that the slugs’ preferred microhabitat was changed by you or your neighbours tidying up in your gardens.
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Could be. They certainly love yard debris, and we’ve got loads of it all year. I’m just not type A enough to go after every leaf and twig. 
While I understand that their slime does provide some protection against UV, it’s got nothing on a hard shell.
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