Note : This no longer works in the official 1.8 release, and from then onward.
My friends and I decided to give the 1.8 PRE-RELEASE update a try. After hours of turning a new world into utter chaos, I found out this glitch. It seems people already know about it, but I wanted to show it in action.
Basically using Bone Meal on a mushroom makes it grow giant. But the block under the mushroom instantly becomes Dirt, no matter what it was when you put the mushroom there. We all know where this is going.
Unfortunately it takes A LOT of room for mushrooms to grow giant, so using this to enter the Void by breaking the Bedrock is likely to be tedious. I used Creative mode before the 2nd clip, to make things shorter by having only one Bedrock block to remove.
At the same time, the 1.8 update allows you to use Creative mode to remove Bedrock blocks anyway.
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While bedrock is intended to be unbreakable, it is still possible to destroy in survival mode...