How can I safely dig straight down?

If you really need to dig down as fast as possible while still being safe from lava dives you can dig a 1x2 hole down.

This is exactly as easy as a 1x1 hole, except for the positioning in the beginning. If you stand squarely between two blocks, mining one won't make you fall until you mine the other, allowing you to see what's below. The picture below shows a good postion, the mouse is also correctly placed so that you don't have to move it to mine both stones.

If you position your pickaxe right on the edge of the two blocks correctly, you'll mine one block after the other and slowly descend into the depths. After mining a block it looks like in the picture below.

If you see a pit or lava instead of another block on the side you just mined, you can throw in gravel or sand to fill it up, or try digging around the hazard.

When you end up with lava below it'll look like this:

To do this safely you'll need to make sure you can stop mining at any second, so no efficiency V diamond pickaxe nor doing this in netherrack with anything better then stone.

Small note: don't mind the weird textured kinds of stone and blocks in the pictures, they are from a couple (different) mods.

Place water at the top of your to be 1x1 hole, place a torch each time you wish to breath, and use Aqua Affinity on your helmet so you'll be able to dig faster.

Stand on a ladder, dig down until you can't hit any more blocks, drop down, and then remove your ladder so that you can place it again.

This will be safer than standing on the block you're mining, but dangerous in that you could lose all your blocks down a ravine or in lava. You may also get stranded if you don't bring enough ladders to get out and run out of pickaxes, so keep that in mind.