Minecraft wiki entry for The_Void, associated text: Teleport below the layer 0 using /tp (<targets>|<destination>|<x>).
Make a hole using /fill ~ 0 ~ ~ ~ ~ air.
Spawn an explosive entity powerful enough to break bedrock (e.g. spawning a creeper using /summon creeper ~ ~ ~ {Fuse:0,ExplosionPower:5000}), and break bedrock with it. (Note: Entities, redstone, and block updates will cease while the blast loads)