Minecraft wiki entry for Things_not_to_do, associated text: Do not dig straight down — this is even more important in the Nether, where one-block thick overhangs above lava or high drops are the rules, rather than the exception. It is best not to dig anywhere close to the player's feet with a diamond pickaxe with any level of efficiency, as netherrack breaks instantly with the said combination. The player should be careful on gravel as well—they may dig one block and discover they were standing on a gravel outcrop over a chasm or worse, the lava sea at the base of the Nether. Safe mining techniques work in the Nether, just as they do in the Overworld. If you have to dig straight down in the nether, at least bring feather falling boots, a totem of undying, and a fire resistance and/or slow falling potion to be safer.