Underdark with ocean access?

[Yora]

A simple question: Is it possible that flooded tunnels in the underdark connect to the ocean or would this cause the entire underdark to fill up with water until it reaches sea level?

[Ashtagon]

If there were a connection to the surface ocean, then the entire underdark would be flooded to sea level.

In a realistic campaign, the entire underdark would be flooded to the water table level, regardless of ocean connections. For most temperate lowland areas, this is about 5-30 metres below ground level (obviously, closer to ground level near rivers).

In a fantasy campaign, there is absolutely no reason not to have underdark oceans or giant lakes, which are entirely disconnected from the surface oceans.

[Creed]

Think about the ocean in "Journy to the Center of The Earth"
Now think that that area is the Underdark. Perfectly pheasable to have a seperate ocean underground.

Hope this helps!

[Athaniar]

Don't the Aboleths live in an Underdark ocean, Glimmersea?

[lightningcat]

The mere idea of the Underdark is unrealistic. That said, as long as you make it seem like it would work, you can have Underdark lakes, rivers, seas, and connections to the ocean. I think that there is a wall of water spell that could be used to permiable vertical barrier to stop water flow.

But Journey to the Center of the Earth and similar books/movies are great resources for Underdark ideas.

It doesn't have to be realistic, just internally consistant.

[hamishspence]

Maybe Journey to the Centre of the Earth (the original Verne book) was where they got the idea from?

Hollow Earth was a D&D setting- but it wasn't quite like that- it might have drawn more from the Tarzan/Pellucidar books.

[Lord Vukodlak]

Think about this, if take a cup turn it upside down and press it to the surface of the water and push it straight down, the air pressure in the cup will actually keep the water from bursting in.
The problem is the water pressure at that depth in the ocean would be to great, however it gives some real basis for your fantasy, especially if the under water tunnel ends up sloping upwards

[Yora]

The real problem is, that the underdark has access to the surface. Take your cup and poke a whole in the base. Then you see what happens. ;)

[Set]

In Golarion, the continued existence of a sea in the underdark is explained at least in part by the existence of 'the braid,' a permanant reverse waterfall created by Aboleths tens of thousands of years ago, bringing water from the lowest levels of the underdark right back up to the surface oceans (and, in older times, being a sort of aboleth super-highway, allowing them easy access to and from the surface oceans).

A less specific variation could simply be portals to the Elemental Plane of Water at ocean level (constantly adding water to the world's surface oceans) and in the 'undersea' (constantly removing water from the subterranean oceans, so that they can never 'fill up' and flood the rest of the underdark).

[Eldonauran]

A less specific variation could simply be portals to the Elemental Plane of Water at ocean level (constantly adding water to the world's surface oceans) and in the 'undersea' (constantly removing water from the subterranean oceans, so that they can never 'fill up' and flood the rest of the underdark).

This ... is a good idea. Use this idea.

[Lord Vukodlak]

The real problem is, that the underdark has access to the surface. Take your cup and poke a whole in the base. Then you see what happens. ;)

The above ground access to the underdark is microscopic compared to the size of the underdark it self. I'm not sure of the effects of a few hundred 10-20ft openings compared to thousands of miles of tunnels.

[Jack of Spades]

Well if you were to assume that any openings above the Underdark have been plugged somehow (possibly the barrier between our plane and that one creates some sort of protomagical seal), then it could be assumed that they are being kept down by air pressure.

[Draconi Redfir]

Behold:

X= rock
O = Ocean
. = air



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Just have a kind of "trap pipe" system in which the entrance to the ocean is lower then the surface of the underground lake, and you should be ok.

[Agi Hammerthief]

Just have a kind of "trap pipe" system in which the entrance to the ocean is lower then the surface of the underground lake, and you should be ok.
the air preassure in that cave is equal to that of the water on the ocean side, at a depth equal to the surface of the lake in the cave.
As soon there is a connection to the surface the cave will flood.

otherwise you need a barrier in the water in which case you have a rapid change of water preasure within the waterway as you'd go from a depth of 5 Os to a depth of 14Os

in a world full of magic not much of a problem, just need to be aware of it

[Yora]

The above ground access to the underdark is microscopic compared to the size of the underdark it self. I'm not sure of the effects of a few hundred 10-20ft openings compared to thousands of miles of tunnels.
Me neither, this is why I ask.

Something like this might work:

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Except that it still would be flooded by rainwater and require a structure of the planetary crust very different from the one of earth.

[Agi Hammerthief]

The above ground access to the underdark is microscopic compared to the size of the underdark it self. I'm not sure of the effects of a few hundred 10-20ft openings compared to thousands of miles of tunnels.you'd have a constant wind blowing out of the openings untill the underdark is flooded, eventually
the above ground access to the underdark is microscopic compared to the size of the underdark it self, but for air you've got a huge outlet taken all openings together.

For purely accademic thinking:
the hugeness of the underdark makes it even worse if you assume there is no connection to the surface or ocean initially and then open a channel to the ocean: the huge volume means there is a lot of air to compress, depending how deep under the ocean the underdark is and where the breach to the ocean is you get massive flooding - 10m of water per 1atm preassure and with lots of space to compress into it takes a lot of flooding to get the preassure up.



<snip model>that looks workable.


Except that it still would be flooded by rainwater and require a structure of the planetary crust very different from the one of earth.
the D&D multiverse is soaked with gods, magic, alchemy etc. there really should be no problem with this little detail.

[Thrice Dead Cat]

Just a minor note, but for the rainwater issues of the Underdark, there are plenty of bizarre, sometimes magical, fungi that could take care of that.