Minecraft wiki entry for Breaking, associated text: In the code, blocks use a hardness value of -1 in order to make blocks unbreakable.
The breaking animation is intentionally programmed by the developers to appear off-center on certain blocks such as chests and signs.[2]
Mining an ender chest with a wooden pickaxe takes the longest time in the game (16.9 seconds) to mine a block with the correct tool, without the effect of Mining Fatigue and other mining speed reduction events.