So after frequently getting lost in the natural tunnels I colonized and mined out, I decided a better idea would be to (instead of building stairs down into the ground) make a 3x3 shaft leading straight downward in a mountainside entrance. I could go up and down via ladders and finding natural caves was easy. The further down I got, the more minerals I would find.
This was working pretty well. I had a nice little settlement built up on an island a ways out to sea. The settlement connected to the mainland (and the mine) by a well lit bridge, which was protected from wandering monsters with a gate. It even had a neat little mine cart track running along side it. I was pretty proud of what I made. That is until I realized infdev wasn't actually bottomless.
As I was expanding the shaft, I started to hit bedrock. I clearly couldn't mine it, so I dug the stone around it.
And thus, Ghoul came plummeting down into a blue abyss, through the crust itself, his assload of iron and expensive tools with him.
Never to be seen again...