Better off to not go in alone if you don't know much about the underground layout. If you have a map, and know of multiple exits, it's less dangerous, on two counts. First, you have the possibility of escape and second you have a greater chance of good air and replenishment of bad air.
Beyond that, after a while studying the older methods of cut-and-fill, false flooring, and split-leveling, you can spot the dodgy stuff and proceed accordingly. Most people die in mines because they fall, and most falls are because they have inadequate light sources. Some die of hypothermia (flooded mines, but I've been in dry mines that got me close), and very few die from rockfall.
But, odds are made better by being prepared. I usually have no less than 3 sources of powered lighting, an O2 meter (which I need to replace soon), a bunch of glowsticks (usually the white ones), a small can of compressed 'pilot' oxygen, gloves, a helmet, good boots, and a stout knife. If I'm going down into a hole, then a descent/ascent system will be built according to proper TRT practices where possible. If not doable, then I'll settle for good mountaineering rope setups as well.
I've been in mines where people used shitty 3/8" Home Depot rope (left behind) for going down winzes as a backup handhold. Crappy knots on crappy anchors = sudden death. If possible, I like to get a truck up close and personal (safely) to the shaft and use it as an anchor. If it's good enough for fire/RS1, it's good enough for me.