Safe way to dig a deep vertical pit?

What's the "safe" to dig a vertical pit?

I tried to channel a 3x3 section straight down. I also put in an up/down stair on the side of the pit but my miners ended up getting stuck, dieing of thirst, and falling and breaking their ankles.

HHH
HHH
HHH
 X 

In my experience, the safest way of doing it is to designate one level for channeling at a time, waiting for the previous one to be completed before you continue.

It requires a bit of micromanagement, but it generally works pretty well, and as long as you have a maintenance chamber at the bottom, you generally don't need the up/down staircase.

Of course, Dwarf Fortress isn't exactly a bastion of OSHA compliance, and mining is a cheap skill to train, relatively speaking, so safety can in many cases be favorably sacrificed for speed.

That said, the absolute fastest way, so long as you have access shafts on each Z-level, is to dig each level, then build a constructed floor above in the shape of the desired pit that is subsequently collapsed using a support.

Just make sure that the pit has solid ground underneath…

Digging a straight up/down stairway is dangerous, because, if any dwarf happens to fall, they'll fall all the ways down, ending in a bunch of red ² at the bottom. And everybody know that crushed dwarf is hard to clean.

I would rather make your pit with an alternated staircase on the side, like this:

HHH   level Z=0
HHH
HHH
>.<


HHH   level Z=-1
HHH
HHH
<.>

Like this, if a miner happen to fall, he will just for for one stair, much less dangerous.

Also, you can dig your staircase first, it won't matter, but make sure to pit the pit from top to bottom, a floor at a time. Since channel automatically create a ramp on the floor below, your dwarves won't get stuck if they happens to fall while digging, if the floor below have not been dug at all.