Sandstone Wall

An Underground Desert generates lined with this wall, but knocking it down leaves nothing to pick up. The only supply is your own crafting, which catches out builders who assume they can quarry a stretch of desert for material.

The generated backdrop lets enemies appear in front of it. Anything you put up by hand does not, which is why re-walling a desert bunker with your own stock is what makes it quiet. The catch runs the other way too: spraying a solution across a finished build flips your placed wall back into the natural spawn-friendly form, so keep the Clentaminator pointed away from anything you care about.

If Corruption, Crimson or the Hallow reaches the desert, the walls repaint into the Ebonsandstone, Crimsandstone or Pearlsandstone version, and the converted wall then pushes the spread onward by itself. Cutting the biome off before it hits a large desert saves a lot of cleanup later.

Throwing the ordinary wall into shimmer returns the Treacherous Sandstone Wall, which looks identical but places the spawn-permitting version. Two reasons to want it: a cave build that still behaves like a cave, and a farm where you want spawns.