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[Quarque]

I embarked on a cold site with an aquifer just below the surface. In winter I was channeling out a channel for fortress defense, when disaster struck. I hit the aquifer and both of my miners fell down the channel they just dug, the water instantly froze, the dwarves were encased in ice and they died. They carried my only pickaxes.

So now it is summer again and the corpses are visibile, together with the pickaxes, in 7/7 water. But my dwarves apparently can't fish them out. I tried building a screw pump, but there is not enough space. And I can't make more space without a pickaxe. I also have no access to any stone, although I do have lots of wood.

What now? Any way out of this mess?

And another question: why did my dwarves fall down their own channel - is there a way to prevent that from happening? According to the wiki it should not be possible, I'm confused about how it happened.

[Zuglarkun]

Wait for the next human or dwarven trade caravan, trade for a pickaxe or just take it.
You can make pump parts out of wood in the meantime, if you want to pump out the water in the aquifer.
I would rather wait for winter to give it another go, be able to retrieve your pickaxes that way too.

If they were standing on the channeled tile while it was being channeled, they would fall. I've had it happen to me before. I could be wrong, but I surmise that is what happened since you had two miners channeling at the same time. Maybe make single designations or priority your designations to give you more control over which tiles get channeled first.

[PatrikLundell]

You say there is not enough room for a screw pump: Do you mean there isn't room physically or that you don't have room for the water to flow off? In the latter case a Portable Drain might be useful, i.e. a track stop set to dump into a wall with a mine cart on top of the track stop and a screw pump pumping into the track stop tile. This should be enclosed on three sides with walls and with the screw pump's impassable (output) tile on the fourth (and the diagonals should be blocked by walls as well). A Portable Drain can get rid of unlimited amounts of water, keeping the level steady at 5/7 in the track stop tile.

Otherwise caravans are an option, as are migrants, provided you've kept the default to have professions enabled. You may then get miner migrants that come with their own tools.

Dorfs have no concept of workplace safety, and unless you keep tight control over them they'll happily channel away tiles under the feet of others, or remove floor in an order that causes cave-ins. When you order dorfs to channel a shaft straight down they follow the order and fall (safely) each tile channeled, until they manage to grab hold of the wall when falling, at which time they climb out of the shaft (to the ire of the manager).
In your case they might not have fallen: they may have pathed through the tile before there was any water on it as the tile is a ramp, to get killed as soon as water entered the tile from the sides. I've had at least one dorf killed by pathing through a ramp shortcut as sloshing magma entered the tile.

[Quarque]

Right, thanks. Buying pickaxes did the tricks, although I screwed up at first because I tried to hard to negotiate and the dwarven caravan said they were "tired of your childish games" and left, so I had to wait for another year

So one year later they're back, I buy the pickaxe (offering them an outrageous profit) and I promptly receive the message that one of my dwarves has gone MIA. Turns out I had forgotten to remove the unsafe channeling designations, and I had already assigned the job of mining to someone, so he immediately picked up the pick and got himself killed with it.

Another year later I bought two new pickaxes and started work on breaching the aquifer. It extends down all the way to the topmost caverns. One of my dwarves died of old age before I had worked my way down, finally getting access to plain stone, so that I may bury the dead. Epic success. 

[Fleeting Frames]

In current version, merchant happiness is pointless. It's always 50%+ profit, no less, no matter the happiness or skills of your broker.

Though got to admit, literally dying from old age before breaching is hilarious.

However, a stone aquifer that extends to the cavern is ...interesting. I've seen this, and my advice is to not dig any aquifer-bearing rock in caverns unless you can deal with it.

[PatrikLundell]

Yes, conglomerate and sandstone can reach a long way down. I've seen it in the first cavern as well, although fortunately that aquifer was not part of my main biome, so I could get down the to cavern without too much trouble. Breaching the cavern through such an aquifer is not a major issue, although smoothing the walls with a hole to the cavern in the floor is a safety issue until dealt with. Any mining in an aquifer is messy, though, but not impossible. Water spilling into a sufficiently large open area is not much of an issue either, as the water spreads out to a maximum, after which evaporation balances the influx.

And, by the way, congratulation to the success. It's good to hear you kept at it despite the difficulties: overcoming such adversities is generally rewarded with a sense of achievement.

[Quarque]

I had a peek at this fortress with DHack Prospect. It turns out I am sitting on top of 10.318 raw Adamantine ore: it should be found from Z:-16 to Z:82.

Wow.. glad I didn't give up on this fortress! Not sure which part of these veins are going to be hollow and which part is safe to mine. But dear Armok, the potential.

[xMAWLx]

You could maybe have used buckets to transfer water into a pit?

[PatrikLundell]

@xMAWLx: If I understand your comment correctly (which may not be the case), you don't understand how aquifers work (if so, search the wiki for a description). There is no way a bucket brigade can move water at the rate an aquifer produces new water. The only things that can keep up are drains, Portable Drains, and screw pumps.

@Quarque: Congratulations. Even if you only mine the candy tiles guaranteed to be safe, you've got a decent supply, but safe candy mining is rather overseer time consuming, as you have to mine only a few tiles at a time and use the results to determine what's safe to mine next. You may have a case of a pillar reaching the second cavern.

[Quarque]

Spoiler (click to show/hide)

It turns out the caverns are all at very high elevations on this map for some reason. The third cavern was at about Z:80 to Z:86, and with exploratory mining I found the top of a vein directly below it. Still searching for any others..

Now to mine it safely. If I understood the wiki correctly, the hollow inner tube has a rectangular shape: a 2x2 region on each Z level, directly on top of each other. Correct? If that is the case, mining around the vein and comparing its shape on each Z level should make it easy to find out what parts are safe to mine.

edit: added spoiler tags

[PatrikLundell]

Spoiler (click to show/hide)

As you say, each hollow level is 2*2 and they have to stack on each other so at least one tile provides a direct access downwards. This means the hollow area can be offset at most one tile in any of the 8 directions. This also means each hollow level is encased in a 4*4 wall, so anywhere you don't have 4 consecutive tiles of candy is safe to mine (but make sure to check both directions.
Thus, you can shear off shell after shell until you reach an unsafe region (can be up to 6*6). Additional input in the form of deduction of the shape of the hole above and/or below can allow you to deduce another piece is safe to shear off.
If you have deduced the 4*4 minimum shape on one level, you can safely remove everything outside a 6*6 region on the level below without further input, but to go beyond that you have to look at where you have candy and where you don't in that 6*6 block's edge. If you're unsure about the size of the safe region on one level, this uncertainty propagates to the level below, where you need an extra layer of safety in the uncertain direction.

[Quarque]

I found an "easier" way:

Spoiler (click to show/hide)

One more miner sacrificed himself, I let him open the tube down the root of the vein on purpose, after walling him off.
This reveiled most of the hollow tube, about 60 levels of it. Above that I will still have to use your method.

A cool detail is that my dwarves decided to make his memorial slab out of pitchblende (I did not pay attention to material). How appropriate!

EDIT: In other news, a section of the top cavern has collapsed. It was nowhere near any of my activities, so it's a mystery how it happened. I had not even seen that part of the cavern yet. It must have had something to do with the aquifer.

Shortly afterwards, the game crashed. I still have a recent savegame (because seasonal autosave).. this might end up as a bug report.

[PatrikLundell]

One way for spontaneous collapses to occur is to have trees collapse as a result of a fire (can be caused either by magma pools or flame bases FBs).

Spoiler (click to show/hide)

One reason the 'peek' method for spire examination is not recommended is that it reveals hell, and that spawns the hordes of hell, which is said to have a severe impact on the FPS(I haven't seen much of an effect, though). If you DO want to use the peek method it's far better to carve a fortification. At most a demon can send a range attack through the fortification to kill the engraver, but they can't get through. You need to have very bad luck for the engraver to be killed, though.