^Just don't dig straight
down. Knocking out the block you're standing on could mean a nasty fall into a dark cave full of monsters, or worse, a pool of lava. Also, make sure you lay down ladders and torches as you go so you can quickly get yourself out of the pit and have sufficient light so that mobs don't spawn in the shaft. A good defence against breaking through into an infested cave is to have a skylight at the top of the shaft along with glass floors. It offers great lighting during the day and has the added bonus of setting skeletons and zombies on fire.