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[Yiab]

I've built myself a trap that needs a significant water input, among the rest of my water distribution system. Basically, I've got a pump stack up to max z-level, followed by a long horizontal trip to the right location, then a 1x1 vertical shaft heading down 17 z-levels. At the bottom of the shaft there's a door on one side which is attached to a lever elsewhere (that also triggers other stuff, but that's irrelevant to this).

Here's the problem: the 1x1 shaft won't fill at all!

The bottom has 7/7 water, but whenever the square above it gets any water, that water just vanishes.

I tried replacing the vertical shaft by a step-shaft - 1 up, 1 sideways - but the same problem keeps happening. I watch the z-level above the bottom, I see 7 water on the bottom level, then I see 3 on my z-level, then it's gone.

What's going on here and how can I fix it?

Miscellaneous information that may or may not be relevant:
My map has "warm" temperature.
My map has a river and borders on the ocean.
The map has salt water, but the water goes through many pumps before it reaches this part of the structure, and only touches artificial walls and floors after entering my water system.
Most of the vertical shaft is made of diorite, but there is some microcline and some cobaltite. The bottom of the shaft is cobaltite.
The door at the bottom of the shaft is made of orthoclase and has 1 orthoclase mechanism in it.
The water passes by standing magma (1 square in between them) 4 z-levels above the bottom of the shaft, but it is never immediately above any magma.
There does not appear to be any in the shaft, but there is plenty of mist.

[Canalan]

Am I correct in assuming that the long horizontal shaft is 7/7 all the way to the drop-shaft?
Continuing as though I am, is this trap running (recieving and removing water) while you are trying to fill the pipe?  If it is, then there is your answer.  If I just gave you the DF equivalent of asking if your computer was plugged in, sorry.
Otherwise, maybe you have a leak in the supply system or pipe.
Also, nice documentation.

[Yiab]

Yes, the horizontal shaft is 7/7 almost the whole way to the vertical one - it's constantly spilling over into the top of the vertical shaft and falling, so the water level is 5 or 6 nearer the vertical shaft. The trap is not one which "runs", it's a triggered trap, and I haven't triggered it at all.
I would think there was a leak somewhere, but there's no place anywhere near the system where there's unexpected water, and there's no aquifers on the map (something I forgot to specify in the OP). Also, when I tried a step-shaft instead of a pure vertical one, I rebuilt the vertical shaft above the bottom level and a small section of the horizontal shaft, and saw the same problem of vanishing water.

I'm currently in the process of clearing the other parts of the trap so I can safely release the water at the bottom level and see what happens, but I'm no optimistic about the results.

Update: The entire step-shaft is also 7/7 water except the one square precisely above the bottom square. The pressure is such that this fills that square to 7/7 immediately, but every few steps it vanishes again (from 7 to 0 in a single step) and is refilled again the next step.

[denito]

Maybe you were unlucky enough to happen to dig the bottom level of your shaft into an aquifer?  That would exhibit the behavior you describe:  always 7/7 water but absorbing anything over.  You can tell if it's aquifer or not by either trying to pump out that bottom tile or digging a larger trench at that level.  If it's normal rock the water should spread out or pump out, but if you get even more water then it's an aquifer.

You can fix your shaft by flooring over the aquifer hole.  In fact, that might be a good test to do first:  what happens if you construct a floor over your 7/7 level of water and try filling the shaft again?

[Yiab]

I removed aquifer tags from all inorganic materials before creating the world, so there cannot possibly be an aquifer on this map. On top of that, I have dug all around the affected area, including the layer below, without water spilling out of the walls, and the entire step shaft is artificially constructed (along with everything else).

That said, you are on to something here - when I open up a wall and floor over the 7/7 water spot, water suddenly starts spilling out after the floor is built. Deconstructing the floor again recreates the problem though.

Have I created a 1-square artificial aquifer?

Update: Opening the door on my 7/7 square does not generate endless water.

Update 2: After emptying the 7/7 square of water, removing the floor now above it, and trying again the problem returns. I'm thinking this is a localized substance-specific black hole.

Update 3: I tried dumping the water on the square north of the black hole instead, but the black hole has moved to the new square!

Update 4: Even after the square below is encased in obsidian and re-excavated, the problem remains. Also, I can't seem to fix it by playing around with dfliquids.

Update 5: I rebuilt the bottom 2 floors of the shaft with sandstone instead (just in case it was a weird material properties problem) but the problem remains.

[Hans Lemurson]

What is the flow-rate produced by your pump-stack?  Can you see little bits of water falling down the shaft?  Do they disappear at any point?

It's possible, but unlikely, that the water is evaporating before it hits the ground.  This would only occur if the flow-rate of the water is patheticly low and a warm environment has boosted evaporation.

What is the lowest point at which you can get water to come out of the shaft?

[Yiab]

I thought it might be evaporation, which is why I tried switching to a step-shaft instead. I've seen it 7/7 water all the way down, though, and it just won't stop vanishing.

Side view:

|7|
|7|
----
(1 tick)
|0|
|7|
----

[Sphalerite]

It is because your fortress is next to an ocean.  There is an odd effect with water near an ocean.  When have any body of water near an ocean, and any part of the water is at the level of the surface of the ocean, any water above that level will vanish.  Basically, water at the level of the surface of the ocean acts like an aquifer and absorbs an infinite amount of water from above.  This appears to be a physics hack to make it impossible to raise the surface of the ocean by pumping water onto it - the ocean will absorb an infinite amount of water pumped into it - but it also happens with unconnected artificial bodies of water not actually connected to the ocean.

[Hans Lemurson]

In that case, I would advise re-locating your tower to a non-ocean biome.  It's easy to find the junction between ocean biomes and land-based ones by their sea-cliffs.

[Yiab]

Wow, that's a neat bug. Good to know that's the case - it also explains why my other vertical tower of water (which is entirely below sea level) isn't experiencing the same problem. Thanks, Sphalerite.

I've already put enough time into this location to make it too annoying to start again somewhere else, so I've developed a workaround to let be do the same traps with pumps and a cistern below ground instead.