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This question already has an answer here:

I plan on making an adventure, and in it they have to go deep underwater, through a tunnel, and out, but does water pressure apply in D&D 4e?

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marked as duplicate by KRyan, Aguinaldo Silvestre, Vylix, Purple Monkey, Mark Wells Oct 19 '18 at 6:27

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There are no rules for water pressure.

The only rules in 4e concerning water have to do with swimming and avoiding suffocating (aka how long can you hold your breath) and any spells or magic items that might aid you in either.

Having a subsystem for water pressure would go against the abstraction level of the game

The lack of water pressure rules is in keeping with the overall design approach to 4e. It would add an unnecessary fiddly-bit to resolving swim and suffocate checks. If you feel its necessary to take into account, I would simply add a difficulty modifier in the range of +1-+5 for the check difficulty, but I would advise that you just not worry about it.

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  • \$\begingroup\$ FWIW, there is an item that implies that water pressure does exist: the Decanter of Endless Water from Mordenkainen's Magnificit Emporium, which specifically discusses using its pressure to help break down a door. But I'd agree that simply adding some sort of additional difficulty, or even making a skill challenge, sounds like a better idea than having a whole pressure subsystem. \$\endgroup\$ – JLan Nov 29 '15 at 17:55
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    \$\begingroup\$ @JLan What you're talking about there is "pressure of a water jet", not ambient pressure of deep water. \$\endgroup\$ – Adeptus Nov 30 '15 at 4:31
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It does if you want it to. There are no rules for it though, so you would have to create something. If the characters are using magic or other device to enable them to breathe or act normally underwater, then it may be better to ignore details of pressure, as they are usually secondary to the effects of drowning and not usually considered in high fantasy.

If you do create a rule for effects of pressure, try to be consistent in applying it during the same adventure.

A very quick ad-hoc ruling might be: At the high pressure below certain depth of the tunnel, an unprotected character takes 5 damage (ongoing) per round until they get into a lower-pressure area. Then list any suitable protections. Outside of round-by-round action that damage might be inappropriately high, so you could reduce that to e.g. 5 damage per minute - technically that would not be ongoing damage anymore, which would interact with some PC abilities differently.

I would advise not to worry unless you want to make a theme out of an intensely pressurised deep area that player characters will be interacting with more than once. A damaging effect would make that area seem a little tougher and more exotic. But just damaging PCs for moving through an area because of a bit of imported realism may not serve any purpose. They should either have a means to avoid the effect, or be trading off time spent taking damage against some advantage.

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The game does not care and has no rules for it, but you as the DM can apply special characteristics to the terrain, like pits filled with electric discharges or upstream lava flows.
Usually, those are counted as traps and experience is awarded for them.

To represent a deep immersion you might have the whole zone deal some damage per round, halving movement, imposing penalties on hits to everyone or apply some penalty to athletic checks done to swim upwards, at least during combat.

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