The Meaning of Life, the Universe, and Everything.
I was thinking of trying a combination of mining in a spiral downwards, but stopping to branch out and carefully plugging up any caves I uncover and keeping the place well lit. I'm also capping the entrance in the overworld with a trapdoor and a building. Will monsters still show up inside the mine?
When I have full on iron gear and enough reserve iron just in case I buy the farm, I'll be looking into it. It's so tempting to go spelunking right now despite being unarmored and weidling a stoned sword...
If you light everything up, no mobs will spawn. Except silverfish.
Don't forget about slimes, which ignore light levels (except on the surface in swamps), although if you keep passages less than three blocks wide only harmless tiny slimes can spawn (other sizes require a 3x3 space, 2-3 high); they also only spawn in certain chunks below y=40 (silverfish can be found anywhere below sea level in Extreme Hills, in clusters similar to ore veins).
Only a few bad things can happen to you while mining:
1) Dont put torches in... you know the rest
2) Silverfish.. with no armor...
3) Falling in lavaaaaaaaaaa......
4) Fall into a ravineeeeeeeeeee......
5) Enderman teleports right in front of you... good riddance me.
thats about it