I learned a long time ago that whenever you're going underground, carry atleast 1 stack of 64 trunks (4 stacks of 64 wood for the space of one) and 1 stack of samplings at around 16 or so, that way you can save face if you get lost, and wont die....
Saved my butt that way (by total accident) a while back when an underground river just a few blocks below the ground took me down all the way to adminium, through complete darkness, into a room where 6 or 7 waterfalls emptied.... ...it was fun!