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[Silfurdreki]

I'm building an underwater fortress, and as such, I have to find a way to drain the ocean. For this purpose I found a map with what basically is a very shallow underwater lagoon, there's a roughly circular 2 z-level deep part of the ocean surrounded with a wall of 1 z-level deep ocean. This is the map: http://www.mkv25.net/dfma/map-5619-islandsoul

Now, I've run into a bit of a problem when I tried out the magma channel yesterday. The plan was to cast an obsidian wall over the deeper parts, and then have it collapse down to the deeper seafloor. The problem is that it doesn't. The obsidian stays suspended under the channel, and because of that, I can't plug the lower z-level so that I can pump out the water afterwards.

Basically, this is my problem:

Code: [Select]

This is what I have before pulling the lever to draw back the bridges:

MMMMMMMMMMM
WWWWWWWWWWW
RRR\WWW/RRR

I want the following after dumping magma:

MMM-----MMM
WWWWWWWWWWW
RRROOOOORRR

Instead, I get this:

MMM-----MMM
WWWOOOOOWWW
RRR\WWW/RRR

Legend:
M = Magma (with bridge under)
W = Water
- = Empty space
R = Rock/seabed
/ \ = Ramps
O = Obsidian

How should I approach this? Will the obsidian still not collapse if I deconstruct the walls on either side of the magma channel? Is the project doomed to fail?

I could really use some advice here, I thought that diagonal connections (across z-levels, even!) to walls didn't give supprt...

[Neruz]

In this case you're probably better off channeling out a 'hole' in the middle of the obsidian cap you have and dropping more magma in, assuming the water under the cap isn't connected to the rest of the ocean it shouldn't rise up and fill in the hole.

[Silfurdreki]

I don't really have an obsidian cap, what I have is magma in a channel that goes above the ocean. It's designed to make a one tile wall to cut off a part of the map to the edges of the ocean.

The water under my obsidian IS connected to the rest of the ocean, for the record 

[Sizik]

It looks like the ramps are keeping it from falling the way you wanted.

[Silfurdreki]

After some testing I've concluded that it's not the ramps lending support, but the seabed tiles are. This presents a problem, because I will not be able to completely shut out the ocean.

I guess I'll have to rely on pumps to hold the ocean out in the ramp tiles...