Author's Note: It's the impossible triangle, a famous optical illusion, in Minecraft form. Built by hand :P. Because of the nature of such an illusion, I only included one screenshot - one at an angle at which the illusion is most convincing. Note to the eagle-eyed: I find that it works better in the full-size picture (read: click zoom).
The in-depth metaphorical explanation:
The three sides of the triangle, here, represent you, the world, and other mobs. They are interconnected, symbolizing the fact that one cannot exist without the others, yet they are interconnected in an impossible way, symbolizing the fact that player, world, and creeper cannot coexist in this dimension. The player relies on the world for resources and gravity, yet scars it and blows it up. He also relies on the creeper for excitement and gunpowder, yet avoids it at all cost. Likewise, the world relies on the player to make the world worthwhile, and relies on the creeper to drive the player away at night, yet both the player and the creeper attack the world relentlessly, morphing and cratering the world endlessly. Finally, the creeper relies on the player to make fragile structures worth destroying, and relies on the world to conceal its position until it is time to strike. But it is the world itself which the creeper hurts in the end, and it is the player which kills the creeper in self-defense. And so the circle (rather, triangle) of love-hate relationships continues on so long as the sun and moon somehow rise and fall on an infinite plane (I always found this last bit quite strange - at what point does the moon cross the infinite plane of the minecraft world, and what happens at that point? And why is the void not lit at night? These are questions for another time.)
I used The VoxelBox Official Creative Texture Pack.