In Minecraft 1.7.2, Dark Oak was added as a wood. This came along with a Positioning Glitch when growing the tree.
When growing the tree in certain way, you are able to break any block below it...
This is how you replicate the glitch:
1. Grow a regular 2x2 Dark Oak Tree
2. Mine the 2x2 section at the base of the tree
3. Place more dark oak saplings, and grow them
4. Usually after a random amount of times, a tree will grow positioning itself in the blocks below
This glitch can enable you to break any block including bedrock.