You probably have not noticed, but all underground structures do not go down to Y:1. They go down to Y:2. Caves and ravines have uncut stone blocks at the second last layer (along with all of the bedrock, Abandoned Mineshafts appear to go no lower than Y:2 and Strongholds generate to Y:3 (mining the floor will bring you to Y:2). Is this a bug, or is it to prevent dungrons from allowing access to the void.

Steps to reproduce:

1. Go into Spectator mode.
2. Look for bedrock caves.
3. Look around and use debug to know what layet the most bottom part is. You will realise that you cannot find a floor of a lava lake on top of the last layer.