Rocky Dirt Wall

These nine patterns cover most of the underground and cavern backdrop, and not one of them drops anything when you break it. The crafted item is how you get to build with the same wall your world is already made of, which is the point: it lets a base blend into the rock instead of announcing itself.

Wall the world generated does nothing to stop enemies appearing in front of it, so a room carved straight out of the cavern is not sealed no matter how solid it looks. Replacing that background with the crafted version is what actually shuts the spawns off, and since the two are identical once placed, nobody can tell you did it.

Rocky Dirt and Rough Dirt read as dirt with stone breaking through, which suits the upper underground. The stone patterns run from the clean Old Stone Wall through to heavily broken Craggy and Fractured versions. Those stone variants are also the ones the evil biomes and the Hallow rewrite as they spread, each mapping onto one specific corrupt, crimson or hallowed pattern. Four of them, Craggy, Fractured, Mottled and Stalactite, do not exist on the old console builds or the 3DS version.