In my setting, there is an area that is usually filled with water at high tide and a land bridge at low tide. One day, a magical wall appeared that does not let water through, but does let through anything else. The shape of the wall is a vertical cylinder (with radius high enough that locally, it as well may be a vertical plane). The wall appeared during hight tide, so at low tide, there is now a vertical water surface.

  1. What is the safest way to cross this wall for a human during low tide in each direction? I can imagine that if a human is partially submerged in the water, then the water pressure would only create force on one of his sides and thus might try to eject him rather forcefully.
  2. How dangerous is it to cross in an "improper" way by a person who has no experience with this wall but is otherwise fit?

I figured when you would swim on the surface, you wouldn't be ejected sideways with a large force, but you will then fall form a really awkward position. On the other hand, trying to swim deep in the water will create a lot of sideways force but probably much more manageable fall. Still, you would be shot across the rough sea bed.

Things you can assume:

  • The water at the border is about 10 feet deep.
  • The floor is your average rocky beach, relatively lifeless, there aren't any large rocks but some of the smaller ones can be pointy.
  • The people crossing it are a (relatively) weak high fantasy heroes in vaguely medieval setting (in fact, it's a low level DnD 5e party, but I am interested more in an real-world physics answer than a 5e mechanics answer, thats why I ask here. Assume Earth gravity, Earth-like salt water, regular physically fit human with no modern gear, also no other magic than whatever created the wall.)
  • The wall behaves the way you'd expect it to without thinking too hard about it, eg. it lets through any liquid that is part of body of a living creature, and also the scenario where the hero gets hurt by a last drop of water that doesn't have anywhere to escape when it's pressed between the hero and the wall just doesn't happen. It also doesn't let through anything that is too small to be visible with a human eye (no pile of microbes/originally dissolved stuff right at the border) and the stuff that's dissolved in water doesn't sediment on it. At any time there aren't any major currents at either side of the wall, even if it would make sense for them to be present.
  • $\begingroup$ Does this wall have 0 thickness $\endgroup$
    – Topcode
    Mar 7, 2022 at 23:23
  • $\begingroup$ Yes the wall is infinitely thin. $\endgroup$
    – Viki
    Mar 7, 2022 at 23:24
  • $\begingroup$ If I were to have a water glass on one side, would the water prevent it from crossing over? $\endgroup$
    – Topcode
    Mar 7, 2022 at 23:36
  • $\begingroup$ I imagined that from the moment when the water in the glass would be separated from the sea wholly by the glass and the wall, the wall would no longer prevent it from crossing over, but if you feel like a different behavior would make for easier/more interesting answer, go for it. $\endgroup$
    – Viki
    Mar 7, 2022 at 23:41
  • $\begingroup$ note the sideways force will only exist when partially submerged sideways, and it won't be that strong. also how wide is the cylinder. $\endgroup$
    – John
    Mar 8, 2022 at 0:41

2 Answers 2


You can get past it using something very buoyant.

Have a hatch in the object that can be opened and closed from a ways away. Tie a rope to the object, preferably with knots in it to climb.

Take the object and throw it over the edge. Very quickly open the hatch and then close it after letting a small amount of water in.

The water will then prevent the buoyant object from being able to cross over, allowing you to climb the rope up to the other side. In order to hold your weight, the object would need to be large, meaning more than one person could sometimes be needed to get it over and then execute the process.

basic diagram This diagram I made in ms paint should explain how it works. The reason a jug wouldn't work is that your weight would push it down and then you couldn't get up. The shown design is simplified.

The danger would not be excessive, it would push you away before you could get far enough in. That said, it would push you away (based on my rough assumptions and calculations) so you probably couldn’t just swim through it.

  • $\begingroup$ I love the idea but you may need to explain it in more detail or with a sketch. a large jug or glass bottle should be enough. you don't need to throw it just push it in at an angle. $\endgroup$
    – John
    Mar 8, 2022 at 0:34
  • $\begingroup$ @John i can make a sketch to better show it. $\endgroup$
    – Topcode
    Mar 8, 2022 at 0:56
  • $\begingroup$ @John hopefully that diagram should explain it better than my words could. $\endgroup$
    – Topcode
    Mar 8, 2022 at 1:19
  • $\begingroup$ My main thought is that if this idea works, then the human body (which is primarily water) would be incapable of passing through the barrier in either direction. If it only applies to seawater, then you're strongly advised not to swallow any seawater before attempting to cross the boundary! That's a great way to experience an agonising death by having your bowels ripped out... $\endgroup$
    – Ruadhan
    Mar 8, 2022 at 12:32
  • $\begingroup$ @Ruadhan This brings to my mind the question of "at what point of the digestive process does swallowed sea water cease to be 'sea water' and become part of the body's own hydration?". Based on the OP's statement "The wall behaves the way you'd expect it to without thinking too hard about it" I'd be inclined to think that just the act of swallowing the water would be enough for the magic barrier to allow it to pass through. But your interpretation does make things more ... complicated ... $\endgroup$
    – Harthag
    Mar 8, 2022 at 16:21


10 feet isn't very deep. It would be a work of an hour or less to build a makeshift construction up the side of the wall. Then lift a boat to the top and put it in the water, where the side of the construction prevents it from slipping over the edge. Then board the boat and row or sail where you want. When you reach your destination (assuming it is still low tide), have two rowers keep the boat from the edge while the others climb down a rope. Then the last two can swim to the edge and jump down to be caught by the people on the ground, possibly in a big blanket.

The construction can be quite simple. You can use the buoyancy of the water to your advantage by building a wooden ladder or A-shaped structure and lean it on the edge of the water, perhaps propped up by inflated bladders. You may even be able to pull this up once you are in the boat and drag it with you to your destination for easy descent.

Lacking access to wood, you pile rocks to get you to the top. Lacking a boat, you could swim, possibly aided by swimming bladders or similar.

If this is a regular occurence, somebody could even have pulled a rope through the wall all the way to the destination. Then it is just a matter of pulling yourself through the wall by the rope. Away from the ends of the wall, this rope would float, propped up by air bladders or air-filled jugs or flasks.

  • $\begingroup$ If you have a boat, Why not just go around? That is: launch the boat on the regular water and mostly ignore the magical pillar of water. Doesn't seem to be any restrictions on the go around option. $\endgroup$ Mar 8, 2022 at 20:53
  • $\begingroup$ @GaultDrakkor: Since the cylinder is described as a wall, I assume that it is barrier that you can't just bypass by boating or swimming around it. If it was that easy, why even bother crossing it? $\endgroup$ Mar 10, 2022 at 13:36

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